Title of Invention

HYDRAULIC VEHICLE BRAKE SYSTEM WITH ANTI-LOCK DEVICE

Abstract

(57) Abstract: The present invention relates to a hydraulic vehicle brake system with a two-circuit brake master cylinder, with two brake circuits in a diagonal allocation for two front-wheel brakes and two rear-wheel brakes, and with an anti-lock device which is incorporated into the brake circuits and which has, for each brake circuit, a return pump with an inlet and an outlet and a first and a second electrically controllable valve which are normally open, the first valve being arranged between the brake master cylinder and the relevant front-wheel brake, and the second valve being connected to the relevant rear-wheel brake, characterized in that the second valves (30, 31; 30b, 31b: 30d. 31d) are arranged between the rear-wheel brakes (11, 12) and the brake master cylinder (2), and in that the rear-wheel brakes (11, 12) are connected to the inlets (49) of the return pumps (32, 33) via throttles (35, 36). PRICE: THIRTY RUPEES

Full Text

The invention relates to a hydraulic vehicle brake system with an anti-lock device. The publication DE 4 422 518 A1 discloses a hydraulic vehicle brake system constituting the pre-characterizing clause, with a two-circuit brake master cylinder, with two brake circuits in a diagonal allo¬cation for two front-wheel brakes and two rear-wheel brakes, and with an anti-lock device which is incorpor¬ated into the brake circuits and which, for each brake circuit, has a return pump with an inlet and an outlet and has a first and a second electrically controllable valve, the valves being designed as normally open valves, the first valve being arranged between the brake master cylinder and the relevant front-wheel brake, and the respective second valve being connected to the relevant rear-wheel brake. In this case, the respective second valve normally connects the rear-wheel brake of a brake circuit to the front-wheel brake of the same brake circuit. The disadvantage of this, in the anti-lock mode, is that a brake pressure in the rear-wheel brake cannot rise higher than a brake pressure in the front-wheel brake. If intention was to increase the brake pressure in the rear-wheel brake above a permissible front-wheel brake pressure, the front wheel would have to be overbraked briefly, thus resulting in possibly inad¬missible brake slip, loss of steerability and excessive tyre wear. A brake pressure in the rear-wheel brake which is higher than the brake pressure in a front-wheel brake is desirable, for example, because, when the accelerator pedal is released, a vehicle engine assigned to the front wheels acts on the front wheels with a braking effects, and this may quickly lead to the risk of front-wheel locking from the moment when the brake pedal is actuated, particularly on snow and black ice. A further reason for desiring a brake pressure in a rear-wheel brake which is higher than the brake pressure in the front-wheel brake is that, on a road with poor grip and on account of the correspondingly lower possible vehicle deceleration, the rear wheels may make a relatively higher contribution to a vehicle deceleration than is possible on a road with good grip. Advantages of the invention The advantage of the hydraulic brake system according to the invention, is that in the anti-lock mode, at least on a road with poor grip, a rear-wheel brake pressure can be at least temporarily higher than a front-wheel brake pressure. Advantageous developments and improvements of the hydraulic vehicle brake system specified in the main claim are possible as a result of the measures listed in the subclaims. One of the advantages of the hydraulic break system according to the invention is that in the anti-lock mode, the brake pressure of the front-wheel can temporarily be set higher than the instantaneous rear-brake pressure, depending on conditions. In addition, there is the possibility, during normal braking, when a slip limit assigned to the rear wheels is reached, to discontinue brake-pressure rises in the rear-wheel brakes by closing the second valves and thereby to ensure that, during braking on a road with very good grip, the front wheels tend to lock before the rear wheels. This makes it possible to avoid installing rear-wheel brake, pressure-regulating valves or rear-wheel brake pressure limiting valves which work in dependence on deceleration or on the rear-axle load. Another advantage of the vehicle brake system according to the invention is that cost-effective 2 / 2 - way valves can be used, and 2 / 2 - way valves from the prior art can be adopted. Yet another advantage of the vehicle brake system is that cost-effective 2 / 2 -way valves of previous design and constructional size can be used, and that a flow resistance, necessary for determining brake-pressure build-up rates in at least one of the wheel brakes, is generated by means of the throttle as claimed. Yet another advantage of the vehicle brake system is that rates of change of brake pressure can be set by means of an electromagnet of the valve located between the brake master cylinder and the relevant wheel brake and, if appropriate, can be varied during an anti-lock mode. For this purpose, the exciting current of the electromagnet is set or regulated. Yet another advantage of the hydraulic vehicle brake system is that, during an increase in the brake pressure in a front-wheel brake, within the rear-wheel brake assigned to the same brake circuit the capacity of the return pump is available in full for the rapid lowering of the rear-wheel brake pressure. Yet another advantage of the hydraulic vehicle brake system is that, by means of only one electromagnet for each wheel brake, for example for each front-wheel brake, rates of change of brake pressure can be set by a variable excitation of the electromagnet. For example, a magnitude of pressure difference between the relevant wheel brake and the brake master cylinder can also be set by setting the exciting current, even if the feed capacity of the return pump were to vary, for example as a consequence of varying pressure differences between the brake master cylinder and at least one of the wheel brakes or as a result of different supply voltages of an electric drive motor of the return pumps. The hydraulic vehicle brake system makes it possible, especially when continuous valves or differential-pressure valves are used, after a first pronounced lowering of brake pressure by means of a high feed capacity of the return pumps, to set the feed capacity of these return pumps lower when only slight changes of brake pressure are necessary as the anti-lock mode continues. This has the advantage of lower energy consumption and less noise generated. This possibility is useful when a vehicle braked sharply on a road section with good grip comes onto an icy road section. Another advantage of the hydraulic vehicle brake system is that in a cost-effective way, it is sufficient for each brake circuit to have a bypass, equipped with a non-return valve, for the rapid lowering of brake pressure in the front-wheel and the rear-wheel brake, when the brake pedal is released. The hydraulic vehicle brake system affords the advantage that, with the return pumps, serving for protection against wheel locking during braking by the actuation of the brake pedal, continuing to be used, traction control by the compensation of excessive drive torque on driveable front wheels is additionally possible by automatic braking. The invention specify different constructive solution features for this purpose, although these are not the only ones which, in the context of anti-lock devices of the so-called return type, are possible as a result of the development for the traction control mode. An actual solution example which can be taken from publication WO 94/08831. Finally, when the second valves are closed, the rear-wheel brake pressure can be lowered to the front-wheel brake pressure when brake pressure is lowered, for example by releasing the brake pedal. The present invention provides a hydraulic vehicle brake system with a two-circuit brake master cylinder, with two brake circuits in a diagonal allocation for two front-wheel brakes and two rear-wheel brakes, and with an anti-lock device which is incorporated into the brake circuits and which has, for each brake circuit, a return pump with an inlet and an outlet and a first and a second electrically controllable valve which are normally open, the first valve being arranged between the brake master cylinder and the relevant front-wheel brake, and the second valve being connected to the relevant rear-wheel brake, characterized in that the second valves are arranged between the rear-wheel brakes and the brake master cylinder, and in that the rear-wheel brakes are connected to the inlets of the return pumps via throttles. Drawing Eight exemplary embodiments of the vehicle brake system according to the invention are represented in the drawing and are described in more detail below. Figure 1 shows a first circuit diagram of the vehicle brake system with the first and second valves in the form of 2/2-way valves, Figure 2 shows a second circuit diagram of the vehicle brake system with first valves in the form of 3/2-way valves, Figure 3 shows a further circuit diagram of the vehicle brake system with first valves in the form of continuous directional valves, Figure 4 shows a further circuit diagram of the vehicle brake system with first and second valves in the form of continuous directional valves, Figure 5 shows a further circuit diagram of the vehicle brake system with first valves in the form of electrically controllable differential-pressure valves, Figure 6 shows a further circuit diagram of the vehicle brake system with first and second valves in the form of differential-pressure valves. Figure 7 shows a representation of a plot of an anti-lock mode on dry asphalt, Figure 8 shows a representation of a plot of an anti-lock mode on a icy road, Figure 9 shows a first development of the vehicle brake system of Figure 1 for the control of traction on driveable front wheels, and Figure 10 shows a second development, likewise for traction control. Description of the exemplary embodiments The hydraulic vehicle brake system 1 according to the circuit diagram of Figure 1 has a two-circuit brake master cylinder 2 with a reservoir 3, a brake pedal 4, a pedal rod 5, a brake booster 6, two brake circuits I and II, two front-wheel brakes 7, 8 with wheel-brake cylinders 9, 10, two rear-wheel brakes 11, 12 with wheel- brake cylinders 13, 14, an anti-lock device 15 and, belonging to the anti-lock device 15, a control unit 16 and wheel-rotation sensors 17, 18, 19 and 20. The brake circuit I includes a main brake line 21 which proceeds from the brake master cylinder 2 and leads to the anti-lock device 15, as well as wheel-brake lines 22 and 23 which proceed from the anti-lock device 15 and terminate at the wheel-brake cylinders 9 and 13. In an identical way, the brake circuit II includes a main brake line 24 and two wheel-brake lines 25 and 26, a wheel-brake line 25 leading to the wheel-brake cylinder 10 and the wheel-brake line 26 to the wheel-brake cylinder 14. The anti-lock device 15 has a valve reception block 27, into which first valves 2 8 and 29 and second valves 3 0 and 31, two return pumps 32, 33 which have a common drive motor 34, two throttles 35, 36 and, in series with these, two non-return valves 37 and 38 are incorporated. Optionally, further non-return valves 3 9 and further throttles 40 can also be incorporated. The first valves 2 8 and 29 are designed as 2/2-way valves controllable by means of electromagnets 41 and 42 and are opened when the electromagnets 41 and 42 are currentless. The second valves 30 and 31 can be con¬trolled by means of further electromagnets 43 and 44 and are likewise designed as 2/2-way valves which are open when the electromagnets 44 and 45 are currentless. At the Same time, the first valves 28, 2 9 are incorporated in the two brake circuits I and II between the main brake lines 21 and 24 proceeding from the brake master cylinder 2 and the wheel-brake lines 22 and 23 leading to the wheel-brake cylinders 9 and 10. The non-return valves 39, in conjunction with bypass lines 45 and 46, form by¬passes round the first valves 28 and 29, the non-return valves 3 9 being openable to the main brake lines 21 and 24 and therefore to the brake master cylinder 2. In this case, the non-return valves 39 can be designed, for example, in the form of the known sleeve-type non-return valves and can be combined with the valves 2 8 and 2 9 to form structural units 47, as represented symbolically by iot-and-dash borderlines. For example, known springless 3 all-type non-return valves or the like can also be incorporated. The further throttles 40 are connected in series with the first valves 28 and 29 and are likewise included in the structural unit 47. The second valves 30 and 31 are incorporated between the main brake lines 21 and 24 and the main brake lines 25 and 26 and therefore between the brake master cylinder 2 and the wheel-brake cylinders 13 and 14. Optionally, once again, the non-return valves 3 9 are arranged with bypass lines 45 and 46 as bypasses leading round the second valves 3 0 and 31 and openable towards the brake master cylinder 2. A throttle 40 is once again connected in series with each of the second valves 3 0 and 31. Once again, the second valves 30, 31 and the further non-return valves 39 and further throttles 4 0 can be combined to form structural units 48. The return pumps 32, 33 have inlets 49 in the form of inlet non-return valves and outlets 50 in the form of outlet non-return valves. The outlet non-return valves 50 are connected to the main brake lines 21 and 24. The inlet non-return valves 49 are permanently connected hydraulically to the wheel brake lines 22 and 25 and therefore to the wheel-brake cylinders 9 and 10. In this case, as already mentioned, the wheel-brake cylinders 9 and 10 are assigned to front-wheel brakes 7 and 8 which belong to the brake circuits I and II. The brake cir¬cuits I and II are accordingly in a diagonal allocation. In contrast to the wheel-brake cylinders 9 and 10 of the front-wheel brakes 7 and 8, the wheel-brake cylinders 13 and 14 of the rear-wheel brakes 11 and 12 are connected to the inlet non-return valves 49 of the return pumps 32 and 33, with the throttles 35 and 36 and the non-return valves 37 and 38 connected in series with the latter being interposed. As a result, when the by¬passes round the second valves 30, 31 are omitted, the rear wheel-brake cylinders 13, 14 can be rapidly relieved of brake pressures in a cost-effective way by the bypasses of the first valves 28 and 29 at the end of braking by the release of the brake pedal 4. If the non¬return valves 37, 38 were omitted, the two bypasses could be placed round the two second valves 30, 31. The wheel-rotation sensors 17 to 20 are designed, for example, in the known way, such that, for each revolution of a wheel assigned to them, they emit a number of pulses which are delivered to the control unit 16 by means of lines 51 shown truncated. The control unit 16 is designed, in a way known per se, such that it can recognize from time intervals between pulses coming from the wheel-rotation sensors 17, 18, 19 and 22 whether there is a risk of wheel locking, that is to say an inadmissibly or adversely high slip. Depending on the recognition of a risk of wheel locking on at least one of the wheels (not shown) , the control unit 16 causes the drive motor 34 to be switched on and the return pumps 32 and 33 thereby put into operation. Furthermore, the control \init 16 controls into the closing position at least that of the first valves 28, 29 or second valves 30, 31, on the associated wheel of which there is the risk of wheel locking. If, for example, there is the risk of wheel locking on a front wheel which is assigned to wheel brake 7 and therefore to the wheel-brake cylinder 9, the first valve 2 8 is closed, with a result that the return pump 32 which is in operation feeds pressure medium out of the wheel-brake cylinder 9 back to the brake master cylinder 2. Consequently, within the wheel-brake cylinder 9, the brake pressure drops, the braking effect decreases and the risk of locking is diminished. When the risk of wheel locking has diminished sufficiently, the first valve 28 is, for example, opened, so that it assumes its ignition position again, with the result that a pressure difference between the brake master cylinder 2 and the wheel-brake cylinder 9, occurring as a result of the preceding lowering of brake pressure, becomes smaller through the throttle 40. In this case, the throttle 40 serves for slowing the reduction of such a pressure difference, so that, on the one hand, the generation of noise caused by the opening of the first valve 28 is as low as possible, and, on the other hand, variations in brake force do not take place abruptly, which could possibly lead to new overbraking of the relevant wheel. A second possibility is to select the cross-section of the throttle 40 larger than for the operating mode described and, instead, during an increase in brake pressure in the wheel-brake cylinder 9, periodically to open and close the first valve 2 8 in a way described in US Patent Specification 3,637,264, so that a brake-pressure rise takes place in a step-like manner. If the risk of wheel locking arises, for example, on a rear wheel which is assigned to the rear-wheel brake 11, the second valve 30 is closed. Consequently, the switched-on return pump 32 will feed pressure medium back out of the wheel-brake cylinder 13, the said pressure medium flowing through the throttle 35 and the non-return valve 37 on its way to the inlet non-return valve 49, through the outlet non-return valve 50 and the main brake line 21 to the brake master cylinder 2. Consequently, the brake pressure drops in the wheel-brake cylinder 13, so that a risk of wheel locking, which has arisen on the rear wheel, is diminished or eliminated. If the risk of wheel locking is eliminated, the second valve 30 can also be controlled into its opening position, for example in a way described for the first valve 28. It can be seen that, within the brake circuit I, the front-wheel brake 7 and the rear-wheel brake 11 each have their own valve 28 or 30, so that the supply of pressure medium from the brake master cylinder 2 to the wheel-brake cylinders 9 and 13 can be controlled individ¬ually in dependence on the risk of wheel locking. Thus, because the rear-wheel brake 11 is assigned its own valve 30 for interrupting the inflow of pressure medium, and because the rear wheel is assigned the wheel-rotation sensor 19, and because the control unit 16 is capable of recognizing increasing wheel slip and an incipient risk of wheel locking, that is to say the occurrence of brake slip and its magnitude, in addition to its use in the anti-lock mode, the second valve 30 is also employed, in the normal braking mode when the brake pedal 4 is actuated increasingly firmly, to close the second valve 30 as early as when a pre-selected brake-slip magnitude threshold stored in the control unit 16 is reached. This affords the possibility of making rear-wheel brakes very effective per se and, nevertheless, of ensuring that on a road with very good grip, up to a legally prescribed deceleration, front wheels lock before rear wheels and a vehicle thus equipped consequently remains controllable. Consequently, from the closing of the second valve 3 0 and, accordingly in the other brake circuit II, of the second valve 31, because the first valves 28 and 29 are opened, brake pressures in the wheel-brake cylinders 9 and 10 in the front-wheel brakes 7 and 8 can still be increased voluntarily by the driver. For example, and this may differ according to vehicle type and load, a brake-pressure rise in the wheel-brake cylinders 13 and 14 is limited to about 25 bar by means of the second valves 30, 31 at the slip magnitude thresh¬old predetermined for the control unit 16. Operating modes of the anti-lock device With reference to the graph of a plot according to Figure 7, which applies to both brake circuits I and II, the operating mode is described in the case of a braking operation on dry asphalt. A rise in the pressure PHZ, commencing at the time 0, can be seen both in the lower half and in the upper half of Figure 7, PHZ being the pressure in the brake master cylinder 2, the said rise being brought about by the actuation of the brake pedal 4. A briefly delayed brake-pressure rise PRADH occurs for rear wheels in the wheel-brake cylinders 13 and 14, for example by means of the throttles 40 preced¬ing the second valves 30 and 31, the said brake-pressure rise being limited to the previously mentioned value of approximately 25 bar according to the above-described design of the control unit 16. Correspondingly, in Figure 7, a line assigned to PRADH runs essentially parallel to the time reference line. In the upper half of Figure 7, a pressure rise PRADV in the wheel-brake cylinders 9 and 10 of the front wheels takes place initially only with a little delay in relation to the pressure PHZ in the brake master cylinder. The brake pressures in the wheel-brake cylinders 13 and 14 and in the wheel-brake cylinders 9 and 10 result in a vehicle deceleration which is represented both in the lower half and in the upper half of Figure 7 by a decrease in a vehicle speed VFZG as represented by inclined straight lines. Starting from a vehicle speed of about 90 km/h, this vehicle speed will lead to the stopping of the vehicle after about 2.7 seconds. Now back to the upper half of Figure 7 and the brake-pressure profile PRAV, commencing at the pressure 0 bar. As already mentioned, the pressure PRADV in the brake cylinders 9 and 10 initially rises with some delay and then more steeply than the pressure PHZ in the brake master cylinder 2, and, as represented by a further line VRADV ruinning like a garland underneath the inclined line VFZG, a gap forms between the two lines VFZG and VRAV which initially increases. The gap indicates the slip of the front wheel relative to the road. The slip of the front wheel increases along the increasing gap. As already mentioned, the control unit 16 recognizes that a risk of wheel locking has arisen for the front wheel or for both front wheels, switches on the drive motor 34 of the return pumps and closes the first valves 28 and 29. Consequently, as can be seen from the line PRADV, the brake pressure in the wheel-brake cylinders 9 and 10 drops, as a consequence of which brake forces are reduced and front-wheel slip diminishes, with the result that the line VRADV approaches the line VFZG in an upward direc¬tion. Because the first valves 28 and 29, are, in principle, two-position valves and the elimination of a risk of wheel locking by a provisional lowering of brake pressure may be followed by a new risk of wheel locking, the wave-like profile PRADV occurs in the wheel-brake cylinders 9 and 10 of the front-wheel brakes 7 and 8 as a result of the repeated lowering of brake pressure and increase in brake pressure. Thus, because brake-pressure increases and brake-pressure reductions alternate with one another, the brake slip will naturally also vary, and this can be seen from the likewise wavy line VRADV in the upper half of Figure 7. Consecpaently, because pressure-medium quantities are extracted from the front-wheel brake cylinders 9 and 10 alternately and are fed back by means of the return pump 32 or 33 to the brake master cylinder 2, from where the pressure-medium quantities can be conveyed back again to the wheel-brake cylinders 9, 10 as a result of the opening of the first valves 28 or 29, pressure fluc¬tuations naturally also occur in the brake master cylin¬der, and this can be seen from the wavy line PHZ. As can be inferred from the profile of the line PRADH in the lower half of Figure 7, no anti-lock mode was necessary for the rear wheels of the vehicle. The anti-lock mode on an icy road can be seen from the graph of the plot according to Figure 8. In contrast to Figure 7, a lower initial speed and a differ¬ent scaling of pressures are represented in Figure 8. If it is assumed that a driver actuates the brake pedal 4 quickly, the pressure PHZ, that is to say the pressure in the brake master cylinder 2, rises quickly. The pres¬sures PRADH in the two wheel-brake cylinders 13, 14 also rise steeply with some delay, so that, before the closing of the second valves 30 and 31, the pressure PRADH in the rear-wheel-brake cylinders 13 and 14 rises so high that rear wheels begin to lock. This can be seen from the steeply descending part line VRADH in relation to the vehicle speed VFZG. The closing of the second valves 30 and 31, with the return pumps 32 and 33 simultaneously being switched on, brings about a steep and big brake-pressure drop in the wheel-brake cylinders 13 and 14 according to the profile of the curve PRADH assigned to the rear wheel. A very pronounced risk of wheel locking also arose at the same time for the front wheels, and this can be seen in the upper half of Figure 8 from an even clearer, bigger drop in the wheel circumference speed VFZG. About 0.25 seconds after the start of braking, the lowest wheel circumference speed of the front wheels is reached as a result of the closing of the first valves 3 0 and 31, with the return pumps 32 and 33 running and with a resulting big reduction in brake pressure. The front-wheel circumference speed VRADV subsequently approaches the vehicle speed VFZG, with the result that the brake slip obviously disappears for a moment and the risk of wheel locking is then or thereby eliminated. The elimination of this risk of wheel locking allows a brake-pressure rise in the wheel-brake cylinders 9 and 10 of the front-wheel brakes 7 and 8, and this can be seen in the upper half of Figure 8. However, the first brake-pressure rise after the deep valley then once again triggers a risk of wheel locking, and this can be seen from the inclined part of the line VRADV immediately after the disappearance of brake slip. Thereafter, by virtue of a repeated closing and opening of the first valves 30 and 31, the brake pressure in the wheel-brake cylinders 9 and 10 of the front-wheel brakes 7 and 8 oscillates about an average value which is of the order of magnitude of 10 bar. In contrast to this, the brake pressure in the wheel-brake cylinders 13 and 14 of the rear-wheel brakes 11 and 12 oscillates about an average value of about 2 0 bar. It can therefore be seen that the average rear-wheel brake pressure is markedly higher than the average front-wheel brake pressure during an anti-lock mode on an icy road, whereas, in the braking mode without the risk of wheel locking, the rear-wheel brake pressure is no higher than the front-wheel brake pressure. As can be seen, this fulfils the desire, men¬tioned in the introduction to the description, of being able to set the rear-wheel brake pressure higher than the front-wheel brake pressure, so that, if appropriate, rear-wheel brakes can make an increased contribution to vehicle deceleration. A hump-like elevation of the pressure PHZ in the brake master cylinder 2 from the moment when the risk of wheel locking arises both on front wheels and on rear wheels can be explained by the fact that the driver has actuated the brake pedal 4 with a specific speed and, as a result of the closing of the first valves and second valves 28, 29/ 30 and 31, suddenly encounters increased resistance, thus leading to a temporary elevation of pressure in the brake master cylinder. The elevation of pressure is assisted by the fact that the return pumps 32 and 33 force pressure medium out of the wheel-brake cylinders 9, 10, 13 and 14 back into the brake master cylinder 2 to a considerable extent. The vehicle brake system la according to the invention, as shown in Figure 2 differs from the vehicle brake system 1 as shown in Figure 1, in that first valves 2 8a and 2 9a, which are arranged in each case between the main brake lines 21 and 24 and the wheel-brake lines 22 and 23, 25, are designed as 3/2-way valves controllable by means of electromagnets 41 and 42. In this case, these second valves 28a and 29a are designed in such a way that they make connections between the main brake lines 21 and 24 and the wheel-brake lines 22 and 25 when the electromagnets 41 and 42 are currentless. When exciting current is applied to the electromagnets 41 and 42, the valves 28a and 29a separate the wheel-brake cylinders 9 and 10 from the brake master cylinder 2 and connect the wheel-brake cylinders 9 and 10 to the respec¬tive inlet non-return valve 49 of the respective return pump 32 or 33. Brake-pressure reduction speeds in the wheel-brake cylinders 9 and 10 can be influenced by incorporating further throttles 40a between the 3/2-way valves 2 8a and 29a and the relevant return pumps 32 or 33. If the 3/2-way valves 28a and 29a are in the basic positions shown and there is a risk of wheel locking on rear wheels, brake pressures in the wheel-brake cylinders 13 and 14 are lowered by closing the second valves 3 0 and 31, without a reduction of the brake pressures in the wheel-brake cylinders 9 and 10 of the front-wheel brakes 7 and 8. As in the exemplary embodiment according to Figure 1, it is possible, without this being shown in Figure 2, once again to combine the first valves and second valves in each case with the non-return valves 39 and the throttles 40 to form structural units and insert them into a valve reception block 27a. On account of the first valves 28a and 29a, which are now designed as 3/2-way valves, and changed ducting 4 0 necessary for this purpose, the valve reception block 27a differs from the valve reception block 27 of Figure 1. The vehicle brake system lb according to the invention, shown in Figure 3, differs from the vehicle brake system 1 according to Figure 1 in that first valves 2 8b and 2 9b are developed into continuous valves, this being symbolized, according to ISO Standards, by parallel strokes next to the squares of the valve symbol and by arrows for variability on associated electromagnets 41b and 42b. The first valves 28b and 29b can therefore be used as throttling directional valves, with the advantage that, as a result of continuous closing by means of increasing current intensity which acts on the electro¬magnets 41b and 42b, the first valves 28b and 29b have an initially throttling and then closing effect. Con¬versely, by a gradual reduction in the current inten¬sities serving for closing the first valves 28b, 29b, gradual opening, along with lessening throttling, can also be achieved. This affords the advantage, known in connection with continuous directional valves, that fluid columns located in the main brake lines 21 and 24 and the wheel-cylinder lines 22 and 25 can be accelerated and decelerated essentially without jolts and therefore with low noise. By varying deexcitation of the electromagnets 41b and 42b, it is also possible, during an anti-lock control mode, to set the rates of change of brake pressure to favourable values, without throttles 40, 40a shown in Figure 2 being exchanged. A further measure with the effect of a reduction in noise is to develop the control unit 16b in such a way that it can reduce the rotational speed of the drive motor 34 of the return pumps 32 and 33 when relatively-low rates of change of brake pressure are sufficient in the anti-lock mode. The vehicle brake system 1c according to the invention, as shown in Figure 4, differs from the vehicle brake system lb of Figure 3 in that, instead of simply designed second valves 30 and 31 of Figure 3, continuous directional valves 3 0b and 31b are now incorporated between the main brake lines 21 and 24 and the wheel-brake lines 23 and 26 and can be identical in design and constructional size to the first continuous valves 28b and 29b. The above-described advantage can thereby also be achieved between the brake master cylinder 2 and the wheel-brake cylinders 13 and 14, The vehicle brake system 1d according to the invention, shown in Figure 5, differs from the vehicle brake system 1 according to Figure 1, in that first valves 28d and 29d are designed as differential-pressure valves, through which the flow passes, for example, in two directions in their basic positions and in which pressure differences between the main brake lines 21 and 24 and the wheel-brake lines 22 and 25 can be set by means of electromagnets 41d and 42d, arrows being marked across the electromagnets 41d and 42d in order to symbol¬ize a variably adjustable magnetic force. The higher the set magnetic force, the greater the pressure difference between the brake master cylinder 2 and the wheel-brake cylinders 9 and 10. By an appropriate design of the control unit 16d, the latter can set brake slip of front wheels in each case to favourable magnitudes via the valves 28d, 29d while the return pumps 32 and 33 are running. Once again, there is the possibility of carry¬ing out a possibly necessary change in the excitation of the electromagnets 41d and 42d, in such a way that jolt¬like flow changes can be largely avoided. As in the exemplary embodiment according to Figure 3, the control unit 16d can be designed in such a way that it sets the rotational speed of the drive motor 34 only insig¬nificantly above that rotational speed which is necessary at that particular moment in the anti-lock mode, depend¬ing on conditions, for the purpose of a variation of brake pressure. The vehicle brake system 1e of Figure 6 differs from the vehicle brake system 1d of Figure 5 in that the second valves 30d and 31d too are now designed as differ¬ential-pressure valves. For example, the second valves 30d, 31d are constructed in the same design and constructional size as the first valves 28d and 29d which are taken from Figure 5. Thereby, advantages resulting from the first valves 2 8d and 2 9d in the anti-lock mode are also achieved in the case of variations of brake pressure in the wheel-brake lines 23 and 2 6 or the wheel-brake cylinders 13 and 14 connected to these. It can therefore be seen that the inventive principle of incorporating an electrically controllable valve between the brake master cylinder and the wheel-brake cylinder of a rear-wheel brake and of connecting the wheel-brake cylinder of this rear wheel brake via a throttle to an inlet non-return valve of a return pump in the relevant brake circuit is possible by means of electrically controllable valves having widely differing designs. The hydraulic vehicle brake system 1f according to the circuit diagram of Figure 9 proceeds from the hydraulic vehicle brake system 1 according to Figure 1 and therefore has for the anti-lock mode, within a valve reception block 27f, first valves 28f and 29f and second valves 30, 31, return pumps 32, 33, throttles 35, 36, non-return valves 37f, 38f and further non-return valves 39. For implementing the traction control mode, the valve reception block 27f additionally incorporates, for each brake circuit I and II, a third electrically con¬trollable valve arrangement 52 and 53 between the brake master cylinder 2 and the respective first valve 28f and 29f and, at the same time, also between the relevant outlet 50 of the respective return pump 32 and 33 and the brake master cylinder 2. Each return pump 32, 33 can thus feed pressure medium both towards the respective first valve 28f or 29f and the respective third valve arrangement 52 or 53. Every third valve arrangement 52 or 53 acts, on the one hand, as a 2/2-way valve which is pressed by a spring 52a or 53a into the basic position shown, which is a throughflow position. The third valve arrangements are each equipped with an electromagnet 54, by means of which the respective third valve arrangement can be changed over to a pressure-limiting function. For this purpose, the symbol for a differential-pressure valve 55 is drawn inside a second square of the symbols for the third electrically controllable valve arrange¬ments 52 and 53. Furthermore, the symbol incorporates a differential-pressure spring 56 which determines the pressure gradient from the outlet of the respective return pumps 50 to the brake master cylinder 2. This pressure gradient is selected in that magnitude which is required as the highest brake pressure for the traction control mode described below. A third valve arrangement acting in this way can be taken from the prior art, for example from publication WO 94/08831. The valve arrangement described in this publication has, on an insertable housing part which merges into a neck-shaped part, a lip gasket which, together with a valve housing block (not shown), forms a non-return valve. The third valve arrangements 52, 53 of Figure 9 can also each be assigned a non-return valve designed in this way, which is represented here by the symbol of a spring-loaded non¬return valve 57 and which forms a bypass round the third valve arrangement 52, the said bypass being openable by means of pressure from the brake master cylinder 2. According to publication WO 94/08831, every third valve arrangement 52, 53 and the respectively associated non¬return valve 57 forms an integral subassembly. Contrary to this design, it is, of course, also possible to incorporate the respective non-return valve 57 separately and, instead of an integral arrangement of a directional valve having the property of a valve limiting brake pressure, to incorporate a separate 2/2-way valve and a separate differential-pressure valve into a valve recep- tion block which in this case is modified. For the said traction control mode, fourth valve arrangements 60, 61 are additionally incorporated into the housing block 27f. According to Figure 9, every fourth valve arrangement contains a first 2/2-way valve 62 and a second 2/2-way valve 63. The first 2/2-way valve 62 has an opening spring 64 and an electromagnet 65 for closing and is therefore a normally open 2/2-way valve 62. The first 2/2-way valve 62 has an inlet 66 which is connected to the relevant wheel-brake cylinder 9 or 10 of a front-wheel brake 7 or 8 and to the relevant non-return valve 37f or 3 8f. Furthermore, the first 2/2-way valve 62 also has an outlet 67, to which an inlet 49 of a return pump 32 or 33 is connected in each case. The second 2/2-way valve 63 possesses a closing spring 6 8 and an electromagnet 6 9 for overcoming the closing force of the closing spring 68. An inlet 70 of the second 2/2-way valve 63 is connected in each case to the brake master cylinder 2 and an outlet 71 of this second 2/2-way valve 63 is connected to the inlet 49 of the relevant return pump 32 or 33. A control unit 16f differs from the control unit 16 according to Figure 1 in that, in addition to re¬cognizing the risk of wheel locking during braking by the actuation of the brake pedal 4, it is additionally designed to recognize excessive traction slip or even the risk of spinning of driveable front wheels which are assigned to the front-wheel brakes 7 and 8. For this purpose, the control unit 16f compares signal trains emitted, for example, by the wheel-rotation sensors 17, 18 with signal trains from wheel-rotation sensors 19, 20 which are assigned to non-driven rear wheels. The device 15f accommodated within the valve reception block 27f is therefore a wheel-slip control device for avoiding excessive wheel slips during braking by the actuation of a brake pedal or during the drive of front wheels. Wheel-slip control to avoid the risk of wheel locking during braking by the actuation of the brake pedal 4 is carried out in a way described with regard to the vehicle brake system 1 according to Figure 1. In this case, the third valve arrangements 52, 53 and the fourth valve arrangements 60, 61 remain in the positions shown and described. On the assumption that a traction slip becomes increasingly greater on that front wheel which is assigned to the front-wheel brake 7, the control unit 16f switches on the drive motor 34 of the return pumps 32, 33, switches on the electromagnet 54 of the third valve arrangement 52, so that the latter is closed, and con¬trols the second 2/2-way valve 63 of the fourth valve arrangement 6 0 into the opening position and the second valve 3 0 and first 2/2-way valve 62 of the fourth valve arrangement 60 into the closing position. As a result of this, the return pump 32, designed to be self-priming for this purpose, is supplied with pressure medium from the reservoir 3 by the second 2/2-way valve 63 of the fourth valve arrangement 60 and by the main brake line 21 and the brake master cylinder 2 and forces this pressure medium through the open first valve 2 8f to the wheel-brake cylinder 9 of the front-wheel brake 7. At the same time, the third valve arrangement 52 prevents the pres¬sure medium from flowing off to the brake master cylinder 2 and into the reservoir 3. Consequently, brake pressure is generated in the wheel-brake cylinder 9, the said brake pressure rising higher the longer the return pump 32 feeds pressure medium. Rising brake pressure in the wheel-brake cylinder 9 ensures that the associated driving wheel is braked increasingly and, at the same time, a braking torque acting on the braked driving wheel begins to compensate an excess of driving torque. In the event of overcompensation, the rotational speed of the front wheel slows in relation to the rotational speed of a, for example, freely rotating wheel of the vehicle, so that the wheel slip decreases. When the control unit 16f establishes that, for example, the wheel slip has diminished to a permissible magnitude, the first valve 28f for example is closed, with the result that brake pressure contained in the wheel-brake cylinder 9 does not become higher. Since, from the moment of the closing of the first valve 28f, the return pump 32 supplies pressure medium in excess, when a pressure difference is reached between the outlet 50 of the return pump 32 and the brake master cylinder 2 in the amount of the projected highest pressure for the traction control mode, the third valve arrangement 52 is opened by overcoming the force of the differential-pressure spring 56, so that pressure medium fed in excess flows to the brake master cylinder 2 and from there is once again available at the inlet 90 of the return pump 3 2 by means of the second directional valve 63 of the fourth valve arrangement 60. Because the brake pedal 4 is not normally actuated in the traction control mode, the brake master cylinder 2 is normally pressureless, so that the differential-pressure valve 55 of the third valve arrangement 52 acts as a safety valve. The result of the "safety valve" effect is that power is converted into energy loss in the differential-pressure valve 55. This is avoidable if the control unit 16f is designed in such a way that, in the traction control mode, the electromagnet 54 of the third valve arrangement 52 is made currentless from the moment of the closing of the first valve 28f, so that the spring 52a opens the third valve arrangement 52 and the return pump 32 thereby feeds pressure medium without an appreciable generation of pressure, that is to say works essentially in the same way as during idling. When a traction slip has become sufficiently low, for example because the driven front wheel comes on to a road area which has a higher coefficient of friction, less or even no brake pressure is required in the wheel-brake cylinder 9 of the front-wheel brake 7. Consequently, the wheel-brake pressure must be lowered, and this can be carried out in the simplest way by the above-described opening of the third valve arrangement 52 and additionally by the opening of the first valve 28f. This happens whenever the brake pedal 4 is not actuated. Should the brake pedal 4 be actuated, which the coming from a pedal-position switch or stop-light switch 80, the said control unit will keep the first valve 28f closed or control it into the closing position, cause the second directional valve 63 of the fourth valve arrange¬ment 60 to return to the closing position and cause the first directional valve 62 of the fourth valve arrange¬ment 60 to be open, so that the return pump 32 receives pressure medium from the wheel-brake cylinder 9 and feeds it back through the brake master cylinder 2 to the reservoir 3 counter to a pressure prevailing in the brake master cylinder 2. As soon as the front-wheel traction slip has thereby decreased sufficiently, the control unit can cause the first 2/2-way valve 62 of the fourth valve arrangement 60 to return to the closing position and essentially simultaneously cause the first valve 28f to return to the opening position, so that a driver, by actuating the brake pedal 4 in the desired way, can generate brake pressure in the front-wheel-brake cylinder 9. Since the rear-wheel brake 11 is also to take effect simultaneously, the control unit 16f will also cause the second valve 30 to return to its throughflow position. Accordingly, that part of the vehicle brake system If which is assigned to the brake circuit I and which is connected to the main brake line 21 is available unrestrictedly for the desired deceleration of the vehicle. The above-described type of wheel-slip control for the adversely increasing traction slip of a front wheel assigned to the front-wheel brake 7 can also be carried out in a similar way for a driveable front wheel which is assigned to the front-wheel brake 8 of the brake circuit II connected to the main brake line 24. The control unit 16f is accordingly designed to control valves in the brake circuit II in an identical way to that described for valves of the brake circuit I. For example, depending on the prevailing traction-slip conditions, the control unit 16f can also supply the wheel-brake cylinders 9 and 10 of the two front-wheel brakes 7 and 8 simultaneously with brake pressure for the reduction of traction slip. The circuit diagram of Figure 9 has differences from the circuit diagram of Figure 1. Thus, for example, the symbols for springs, present in Figure 1, are absent in non-return valves 37f and 38f of Figure 9. In contrast to a throttle 40 arranged separately and at the same time in series on the first valve 28, the first valve 2 8f and the other first valve 2 9f are designed to act themselves as a throttle 40f, and this can be achieved, for example, by adjusting the width of a valve gap when the valve 28f is open. For this purpose, for example, means which limit armature strokes and which are not shown are adjusted in a way known per se. On the other hand, throttles 40 can of course, also be arranged in series with the first valves according to Figure 1. These first valves would then once again acquire a designation "28" according to Figure 1. The omission of the springs from the non-return valves 37f and 3 8f makes it possible, when the second valves 30 and 31 are closed, for wheel-brake pressures in the wheel-brake cylinders 13 and 14 of the wheel brakes 11 and 12 to be capable of being lowered to those wheel-brake pressures which prevail in the wheel-brake cylin¬ders 9 and 10 of the front-wheel brakes 7 and 8. It can be seen from this that, for example, if the first valve 28f or 29f is closed to adverse effect by means of the non-return valves 39 which are parallel, pressure-medium quantities flow through the non-return valves 3 9 when the brake pedal 4 is released, so that wheel-brake pressures disappear in a desired way. Figures 3, 4, 5 and 6 disclose that first and second valves 28, 29, 30 and 31 of the 2/2-way valve type, which are shown in Figure 1, can be sxibstituted, for example, by continuously adjustable directional valves or by continuously adjustable differential-pres¬sure valves. The valve arrangements 52, 53, 60, 61 serving for traction control are therefore not restricted to combination with those valve arrangements according to Figure 9 serving for anti-lock protection, but the valve arrangements 52, 53, 60 and 61 can also be combined with the vehicle brake systems according to Figures 3 to 6. A further exemplary embodiment of a vehicle brake system 1g serving for wheel-brake slip control during braking by the actuation of the brake pedal and also for compensating excessive drive torque on driveable wheels is shown in Figure 9. This vehicle brake system 1g like¬wise proceeded from the vehicle brake system 1 according to Figure 1, third valve arrangements being identical to the already described third valve arrangements 52 and 53 of Figure 9. However, fourth valve arrangements 60g and 61g differ from the fourth valve arrangements 60 and 61 of Figure 9 in that no electromagnets are required for controlling the fourth valve arrangements 60g and 61g. The fourth valve arrangements are therefore designed as 3/2-way valves 73 which each have a spring 74, deter¬mining a basic position, and a control inlet 75. In each case a control duct 76 leading to the respective control inlet 7 5 communicates with the brake master cylinder 2 through a valve reception block 27g and via the main brake line 21 or 24. An actuation of the brake pedal 4 with the result that pressure medium emerges from the brake master cylinder 2 ensures, by means of the main brake line 21, the control duct 7 6 and the control inlet 75, that the spring 74 is compressed. Every fourth valve arrangement 6 0g, 61g has a first connection 77, a second connection 78 and a third connection 79. The result of this, along with the above reference to the compression of the relevant spring 7 9 brought about by the supply of pressure medium into the control inlet 75, is that the fourth valve arrangement 60g or 61g of Figure 10 is a so-called 3/2-way valve. The first connection 77 of each of the fourth valve arrangements 60g and 61g is permanently connected hydraulically to the brake master cylinder 2 by means of the respective main brake line 21 and 24. The second connection 78 is in each case connected permanently to a wheel-brake cylinder 9 or 10 of a front-wheel brake 7 or 8. The third connection 79 may be designated as a valve outlet and is connected to the inlet 49 of the relevant return pump 32 or 33. Thus, according to Figure 10, when the brake pedal 4 is not actuated, there is a hydraulic connection proceeding from the reservoir 3 through the brake master cylinder 2, through the respective main brake line 21 or 24 and through the respective first connection 77 and the third connection 7 9 to the relevant return pump 32 or 33, so that, after the drive motor 34 has been switched on, the said return pumps can supply themselves with pressure medium from the reservoir 3. In contrast, if control pressure is introduced into the relevant control inlet 75 as a result of the actuation of the brake pedal 4, then, as already mentioned, the respective fourth valve arrangement 60g or 61g changes over to the switching position, with the result that the respective first connection 77 and therefore also the reservoir 3 are separated from the relevant return pump 32 or 33 and, instead, the relevant wheel-brake cylinder 9 or 10 is connected to the return pump 32 or 33 by means of the respective second connection 78. A control unit 16g belonging to the vehicle brake system Ig differs from the control unit 16f of the exemplary embodiment of Figure 9 in that the control unit 16g now has to control only the third valve arrangements 52 and 53 taken over from the exemplary embodiment of Figure 9. To that extent, a cost reduction in terms of electronics and electrics is obtained in comparison with the exemplary embodiment of Figure 9. When the brake pedal 4 is actuated for the purpose of braking, the respective fourth valve arrange¬ment 60g or 61g is controlled hydraulically into the switching position in the way already described, with the result that the inlets 49 of the return pumps 32 and 33 communicate with the associated wheel-brake cylinders 9 and 10, so that, if the brake pedal 4 is actuated too vigorously and a risk of wheel locking therefore arises, the return pumps 32 and 33 can feed pressure medium, contained in excess in the wheel-brake cylinders 9 and 10, back to the brake master cylinder 2 for the purpose of lowering the brake pressure. For this purpose, the drive motor 34 is switched on in the way already described, and at least one respective first valve 28g or 29g is controlled into the blocking position, irres¬pective of which of the front wheels assigned to the front-wheel brakes 7 and 8 is at imminent risk of lock¬ing. In this case, the third valve arrangements 52 and 53 are in their basic positions which, as already described, are determined by springs 52a and 53a. If there is a risk of wheel locking on at least one of the rear wheels, to which the rear-wheel brakes 11 and 12 are assigned, then, as described with regard to the first exemplary embodiment according to Figure 1, at least one second valve 30 or 31 is controlled into the closing position, depending on the conditions, with the result that an inflow of pressure medium through the said valves into the wheel-brake cylinders 13 and 14 of the rear-wheel brakes 11 and 12 can be prevented and, accordingly, the return pump 32 or 33 can extract pressure medium from the relevant wheel-brake cylinder 13 or 14 through the respective associated throttle 3 5 or 3 6 and the non¬return valves 37f or 38f and force it back to the brake master cylinder 2, until the risk of wheel locking on the respective rear wheel disappears. Traction control mode generally occurs when the brake pedal 4 is not actuated, because a driver controls the drive torque for the driveable front wheels by means of an accelerator pedal (not shown). Consequently, every fourth valve arrangement 60g and 61g is in the basic position shown. Should traction slip increase inadmissibly, for example on a driven front wheel belonging to the front-wheel brake 7, the control unit 16g recognizes this risk by virtue of trains of signals from the associated wheel-rotation sensor 17. As a consequence, the control unit 16g will switch on the drive motor 34 of the return pumps 32 and 33 and, as in the exemplary embodiment according to Figure 9, control the third valve arrangements 52 and 53, so that they act as required differential-pressure valves for the purpose of limiting a maximum pressure gradient between the outlets 50 of the return pumps 32 and 33 and the main brake lines 21 and 24. With the first valve 2 8g still open, the return pump 32 will pump pressure medium, extracted from the main brake line 21 and the reservoir 3, into the wheel-brake cylinder 9, so as thereby to generate wheel-brake pressure and wheel braking torque which at least compensates an excess of driving torque on the front wheel which has led to the triggering of the traction control. The traction slip is thereby reduced. If the traction slip is reduced suff¬iciently, for example the first valve 28g can be closed, so that there is no longer any further rise in brake pressure in the wheel-brake cylinder 9. When the control unit 16g recognizes that the traction slip is falling below a critical threshold, it can cause the third valve arrangement 52 to return to the basic position shown, with the result that pressure medium flows through the non-return valve 3 9 out of the wheel-brake cylinder 9, through the third valve arrangement 52 and the main brake line 21 and through the brake master cylinder 2 back into the reservoir 3. This can, for example, take place while the first valve 28g is still closed. A full relief of the wheel-brake cylinder 9 from wheel-brake pressure is achieved by switching the first valve 2 8g back into the basic position shown. In addition to the above-described way of gener¬ating wheel-brake pressure in the wheel-brake cylinder 9 for the purpose of keeping constant and lowering the wheel-brake pressure, there is also the possibility, in view of the permanent connection present between the reservoir 3 and the inlet 49 to the return pump 32 when the brake pedal 4 is not actuated, of controlling a brake-pressure rise and brake-pressure reductions in the wheel-brake cylinders 9, with the first valve 28g open, by shifting the third valve arrangement 52 back and forth between the basic position shown and that position which results in the functioning of a differential-pressure regulating valve. Then, when the third valve arrangement is shifted into the second position, brake-pressure rises are generated in the wheel-brake cylinder in each case by the return pump 32 and, when the control unit 16g has recognized a sufficient compensation of excess driving torque, the third valve arrangement 52 is returned to the basic position which is determined by the spring 52s. In this method of controlling the third valve arrangement 52 for the purpose of controlling traction slip, brake-pressure rises and brake-pressure reductions succeed one another with sawtooth-like brake-pressure profile. Such a sawtooth-like profile of the brake pressures is typical of a so-called two-position controller which can be produced in a relatively simple way. The above-described traction-slip control mode, brought about simply by switching on the drive motor 34 of the return pump 32 and by shifting the third valve arrangement 52 back and forth by means of the spring 52a or the electromagnet 54 taken from the example of Figure 9, affords the advantage that the return pump 32 merely has to generate the pressure which is required for compensating excess driving torque. Such a wheel-brake pressure can be substantially lower than that pressure difference which the third valve arrangement 52 generates relative to the pressure in the brake master cylinder 2, with the brake pedal 4 not actuated, when the electro¬magnet 54 is switched on. As can be seen, in such a favourable situation, less energy is expended for driving the return pump 32 and also less noise is generated. It goes without saying that an excess of driving torque, to be compensated by means of the front-wheel brake 8 of the brake circuit II and occurring on an associated front wheel (not shown), can be executed in the same way as for the driving wheel assigned to the brake circuit I and to the front-wheel brake 7. The traction-slip control mode can arbitrarily take place, for example, simultaneously in both brake circuits I and II or alternately or by overlapping one another. The exemplary embodiments according to Figures 9 and 10 show that, for fourth valve arrangements, two different paths for pressure medium to the inlets 49 of the return pumps 32 and 33 must be switched. This is achieved, in the example of Figure 9, by the provision of two closeable valve seats, allocated to the two 2/2-way valves 62, 63 in a way not shown, for each brake circuit or for one of the return pumps 32 or 33 in each case. In the exemplary embodiment according to Figure 10, the fourth valve arrangement 60g or 61g has three connections 77, 78 and 79, and in the design as seat-valve arrange¬ ments, according to the prior art, one valve seat (not shown) belongs to the first connection 77 and a second valve seat (not shown) belongs to the second connection 78. It can be seen from this that directional valves of 3/2-way valve type or combinations in each case of two 2/2-way valves can selectively be used as fourth valve arrangements 60g and 61g. Furthermore, it can also be seen that the fourth valve arrangements can be designed so as to be controllable either hydraulically or electro- magnetically. The result of this in turn, for the average person active in the field of wheel-slip control devices, is that, to perform the function of the fourth valve arrangements, 2/2-way valves can be controlled hydraulically or a 3/2-way valve can be controlled electromagnetically. Accordingly, the development of anti-lock devices according to Figures 1, 3, 4, 5 and 6 is not restricted to those third and fourth valve arrangements which are shown in Figures 9 and 10. In contrast to Figure 1, Figures 9 and 10 show the non-return valves 37f and 38f without springs. The non-return valves 37f and 38f are therefore, for example, designed as seat valves with valve balls which, for example by their own weight, can close the valve seats. Since, as is known, such valve balls are very small and therefore light within vehicle brake systems, the valve seats could also be incorporated above the balls, so that a pressure gradient, for example from the front-wheel-brake cylinder 9 to the rear-brake cylinder 13, causes a flow which lifts the valve ball into the valve seat. It is then thereby possible, even in the traction-slip control mode, essentially to keep away from the rear-wheel-brake cylinder 13 a front-wheel brake pressure which is necessary for compensating excess driving torque and which takes effect in the front-wheel-brake cylinder 9. In contrast to the non-return valves 3 9 shown with springs in Figures 1 to 6 and 9, non-return valves 3 9g without springs are shown in Figure 10. Valve seats of the non-return valves 3 9g are preferably arranged underneath the closing bodies designed, for example, as balls. Since the springless non-return valves 3 9g can be opened by means of lower pressure gradients, brake pressures of the wheel-brake cylinders 9, 10 can be lowered further than in the previously described exem¬plary embodiments when the first valves 28g, 29g are closed. This is advantageous when driving on ice and snow and can also accelerate the reduction in brake pressure. The said advantage can, of course, also be transferred to the exemplary embodiments of Figures 1 to 6 and 9. WE CLAIM; 1. A hydraulic vehicle brake system with a two-circuit brake master cylinder, with two brake circuits in a diagonal allocation for two front-wheel brakes and two rear-wheel brakes, and with an anti-lock device which is incorporated into the brake circuits and which has, for each brake circuit, a return pump with an inlet and an outlet and a first and a second electrically controllable valve which are normally open, the first valve being arranged between the brake master cylinder and the relevant front-wheel brake, and the second valve being connected to the relevant rear-wheel brake, characterized in that the second valves (30, 31; 30b, 31b; 30d, 31d) are arranged between the rear-wheel brakes (11, 12) and the brake master cylinder (2), and in that the rear-wheel brakes (11, 12) are connected to the inlets (49) of the return pumps (32, 33 ) via throttles (35, 36). 2. The hydraulic vehicle brake system as claimed in claim 1, wherein a non-return valve (37, 38) openable in the direction of the inlet (49) of the return pump (32, 33) is incorporated in each case in series with the throttle (35, 36). 3. The hydraulic vehicle brake system as claimed in claim 1 or 2, wherein at least one of the valves (28, 29, 30, 31) is a 2 / 2 - way valve. 4. The hydraulic vehicle brake system as claimed in claim 3, wherein a further throttle (40) is connected in series with the 2 / 2 - way valve (28, 29, 30, 31). 5. The hydraulic vehicle brake system as claimed in claim 1 or 2, wherein at least one of the valves is a continuous directional valve (28b, 29b, 30b, 31b). 6. The hydraulic vehicle brake system as claimed in claim 1 or 2, wherein at least the valves located between the brake master cylinder (2) and the front-wheel brakes (9, 10) are 3 / 2 - way valve (28a, 29a). 7. The hydraulic vehicle brake system as claimed in claim 1 or 2, wherein at least two of the valves are differential-pressure valves (28d, 29d, 30d, 31d), the electromagnets (41d, 42d) of which can have a variable exciting current applied to them. 8. The hydraulic vehicle brake system as claimed in claim 1 or 2, wherein a control unit of the anti-lock device (2, 2a, 2b, 2c, 2d, 2e) is provided for controlling the feed capacity of the return pumps (32, 33). 9. The hydraulic vehicle brake system as claimed in claim 1, wherein bypasses (45, 46) with non-return valves (39) openable towards the brake master cylinder (2) are connected in parallel with the first or second valves (28, 29; 28b, 29b; 28d, 29d). 10. The hydraulic vehicle brake system as claimed in claim 2, wherein for the control of traction slip of driving wheels assigned to the front-wheel brakes (9, 10), there is incorporated between the brake master cylinder (2) and the respective first electrically controllable valve (28, 29; 28b, 29b; 28d, 29d; 28f, 29f; 28g, 29g) as well as the brake master cylinder (2) and a respective outlet (50) of the relevant return pump (32, 33) a third electrically controllable valve arrangement (52, 53) which is normally open and which is closed at least to a limited extent in the traction-slip control mode, and in that a fourth valve arrangement (60, 61; 60g, 61g) is incorporated between the brake master cylinder (2) and an inlet (49) of each return pump (32, 33) and a front-wheel brake (7, 8) and the respective inlet (49) of each return pump (32, 33), for connecting the respective front-wheel brake (7, 8) to the respective inlet (49) of the relevant return pump (32, 33) in the braking mode brought about by means of the brake pedal (4) and for connecting the brake master cylinder (2), combined with a reservoir (3), to the respective inlet (49) of the relevant return pump (32, 33), whilst at the same time separating the respective front-wheel brake (7, 8) from the respective inlet (49) of the relevant return pump (32, 33). 11. The hydraulic vehicle brake system as claimed in claim 10, wherein the fourth valve arrangement (60, 61) contains a normally closed, electrically controllable 2 / 2 -way valve (63) between the brake master cylinder (2) and the inlet (49) and a normally open, electrically controllable 2/2 - way valve (62) between the respective front-wheel brake (7, 8) and the inlet (49) of the relevant return pump (32, 33). 12. The hydraulic vehicle brake system as claimed in claim 10, wherein the fourth valve arrangement (60g, 61g) is a hydraulically controllable 3 / 2 - way valve (60g, 61g, 75, 77, 78, 79, 74) which, in its basic position determined by a spring (74), connects the brake master cylinder (2) to the inlet (49) of the relevant return pump (32, 33) and simultaneously separates the respective front-wheel brake (7, 8) from the inlet (49) and which has a control inle(25) connected to the brake master cylinder (2) in such a way that, when the brake pedal (4) is actuated and pressure is thereby generated in the brake master cylinder (2), the respective front-wheel brake (7, 8) is connected to the associated inlet (49) of the associated return pump (32). 13. The hydraulic vehicle brake system as claimed in any one of claims 10 to 12, wherein the third valve arrangement (52, 53) is a seat valve and has a differential-pressure spring (56) between a valve-seat closing member and an electromagnet (54). 14. The hydraulic vehicle brake system as claimed in any one of claims 10 to 13, wherein the non-return valve, which is arranged in each case between a wheel-brake cylinder (13, 14) of a rear-wheel brake (11, 12) and he associated first valve (28f, 29f; 28g, 29g) and which can be opened towards the respective first valve (28f, 29f; 28g, 29g), is a springless non-return valve (37f, 38f). 15. A hydraulic vehicle brake system with a two-circuit brake master cylinder, substantially as herein described with reference to the accompanying drawings.

Documents:

1466-mas-1996 abstract.pdf

1466-mas-1996 claims.pdf

1466-mas-1996 correspondence -others.pdf

1466-mas-1996 correspondence -po.pdf

1466-mas-1996 description (complete).pdf

1466-mas-1996 drawings.pdf

1466-mas-1996 form-1.pdf

1466-mas-1996 form-26.pdf

1466-mas-1996 form-4.tif

1466-mas-1996 petition.pdf