GB2244772A

GB2244772A - A hydraulic braking system

Info

Publication number: GB2244772A
Application number: GB9110819A
Authority: GB
Inventors: Alfred Sigl; Wolf-Dieter Jonner; Georg Spalding
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1990-05-23
Filing date: 1991-05-21
Publication date: 1991-12-11
Also published as: GB9110819D0; JPH04231252A; DE4016560A1

Abstract

A hydraulic braking system having anti-skid (ABS) and drive slip control (ASR) for a motor vehicle has a charging valve integrated into the master cylinder (15), which, for providing brake pressure in ASR operation, connects the inlet line (47) of at least one return pump element (29) to the brake fluid reservoir (18) while a change-over valve (44) breaks a connection between the output line (26) of this pump element (29) and the master cylinder (15) and connects the pump instead through a pressure limiting valve (48) to the reservoir (18). The changeover valve may comprise a spool cooperating with a port in the cylinder wall connected to the pump line (47) or may be a tilting valve or a ball seat valve actuatable by a piston in the cylinder. On initial ASR action an accumulator (45) supplies fluid to the brake via a switching valve (46) and the changeover valve (44). The switching valve also serves for charging the accumulator (45) from the pump. For a diagonal circuit arrangement an extra change-over valve (44) is provided. <IMAGE>

Description

-1DESCRIPTION A HYDRAULIC BRAKING SYSTEM

The invention relates to hydraulic braking systems having an anti-skid system (ABS) and drive slip control (ASR), for motor vehicles.

In a known braking system of this type (DE 38 16 073 A1), having a front axle/rear axle brake circuit configuration and a charging valve for connecting a pump element to a reservoir in the event of ASR, the charging valve comprises a pressure-controlled twoport, two-position valve, whose hydraulic control inlet is connected to a brake circuit outlet of the master brake cylinder. The two-port, two-position valve is connected as a separate valve in the feed line between the brake fluid reservoir and the inlet of the pump element which is associated with the brLke circuit of the drive wheels. On brake pedal actuation. the charging valve is switched over and closes off the feed line, such that, in the event of ABS operation. no brake fluid can be sucked out of the brake fluid reservoir and delivered into the brake circuit.

According to the present invention, a hydraulic brake system having an ABS and ASR, for a motor vehicle, comprises a master brake cylinder, having at least one brake circuit outlet, for delivering a brake -2pressure upon actuation of a brake pedal, a brake fluid reservoir, which is connected to the master brake cylinder, and a hydraulic assemblage, which is connected to the at least one brake circuit outlet of the master brake cylinder and is disposed in advance of at least one wheel brake cylinder of a motor vehicle wheel, and which comprises a charging valve and at least one changeover valve and, by means of the charging valve, at least in the event of drive slip control, connects at least one pump element of a return pump, which is associated with a brake circuit having at least one drive wheel and is self-priming, to the brake fluid reservoir, and, at least in the event of brake pedal actuation, interrupts this connection, and, by means of the at least one changeover valve which, in the event of drive-slip control, isolates the at least one pump element, which belongs to a brake circuit having at least one drive wheel, from the master brake cylinder, and connects it by way of a pressure-limiting valve to the brake fluid reservoir, in which the charging valve is integrated in the master brake cylinder and has a valve opening which is controlled by a valve element and which lies between the brake fluid reservoir and a further outlet of the master brake cylinder, and the valve element is coupled to the brake pedal, and in which an intake j -3line leads to the at least one pump element of the return pump, which is associated with a brake circuit having at least one drive wheel and is self-priming, and is connected to the further outlet of the master brake cylinder.

This has the advantage that a separately assembled, pressure-controlled valve having a closing function on brake pedal actuation is dispensed with. By virtue of integration into the master brake cylinder, the function of the charging valve can additionally be substantially improved in that the valve opening cross section of the charging valve, which is advantageously in the form of a spool valve,, can be made bigger, and closing of the valve even occurs at a brake pressure of less than 1 bar delivered by the master brake cylinder.

Whilst retaining the charging valve integrated in the master brake cylinder, the hydraulic assemblage for brake pressure supply to the wheel brake cylinders may be designed in many ways. For large-volume wheel brake cylinders, which require large volumes of brake fluid to apply the brake calipers, it is advantageous if, in accordance with one embodiment of the invention, a low-pressure accumulator is provided, which, on commencement of ASR operation, feeds by way of the change-over valve and the control valves of the -4slipping drive wheels directly into the wheel brake cylinder thereof. As soon as the pressure of the lowpressure accumulator is lower than the brake supply pressure generated by the return pump, an electromagnetic switching valve, which is disposed between the low-pressure accumulator and the changeover valve, switches to the closed position and isolates the low-pressure accumulator from the brake supply circuit. This prevents the low-pressure accumulator withdrawing brake fluid from the brake supply circuit for the purposes of filling during brake pressure build-up in the wheel brake cylinders. As soon as the brake fluid requirements of the wheel brake cylinders have been met, and a sufficiently high brake pressure has been built up in the wheel brake cylinders, the switching valve switches over again, such that the accumulator is again connected to the brake supply circuit and can be recharged with excess brake fluid. Brake fluid volumes exceeding this are returned by way of the pressure- limiting valve to the master brake cylinder and pass out of its cylinder chamb&r, which is connected to the further outlet of the master brake cylinder, into the brake fluid reservoir.

In accordance with a preferred embodiment of the invention, the switching valve between the low- -5pressure accumulator and the change-over valve comprises a three-port, three-position solenoid valve with spring return, and, in its one valve position, closes off the low-pressure accumulator and bypasses the pressure-limiting valve, in its second valve position, closes off the low- pressure accumulator and interrupts the bypass of the pressure-limiting valve, and, in its third valve position, connects the lowpressure accumulator to the change-over valve. This embodiment of the switching valve has the advantage that, in the event of brake pressure reduction during ASR operation, the pressure-limiting valve can be bypassed, and thus low pressures can be achieved in low-pressure accumulator chambers connected to the pump element, as a result of which it is possible to achieve a substantially more rapid transition to ABS operation.

The invention is further described, by way of example, with reference to the accompanying drawings, in which:

Fig.1 is a block diagram of a dual-circuit braking system having front/rear brake circuit configuration, an anti-skid system and drive slip control, for a passenger vehicle; Fig.2 is a schematic longitudinal section through a master brake cylinder of the dual-circuit braking -6system of Fig.1; Fig.3 is a block diagram of a dual-circuit braking system having diagonal brake circuit configuration; and Fig.4 is similar to Fig.2 but shows a master brake cylinder in accordance with a further embodiment.

In the hydraulic dual-circuit braking system shown in Fig.1 and having front/rear or front axle/rear axle or so-called black/white brake circuit configuration, an anti-skid system (ABS) and drive slip control (ASR), also called pre-drive control, for a passenger vehicle, wheel brake cylinders 10 of the drive wheels 11,12 are disposed in one brake circuit and wheel brake cylinders 10 of the nondriven wheels 13,14 are disposed in the other brake circuit. In general, in such case the drive wheels 11, 12 are the rear wheels of the passenger vehicle. The dualcircuit braking system includes, in a known way, a master brake cylinder 15, which has two separate brake circuit outlets 16,17 for connecting one each of the two brake circuits, and is connected to a brake fluid reservoir 18. On actuation of a brake pedal 19, an equally large brake pressure is delivered to each of the two brake circuit outlets 16,17 in the brake circuit.

The dual-circuit braking system further includes a four-path hydraulic assemblage or aggregate 20, which has four outlet passages 21 to 24 and two inlet passages 25 and 26. A respective wheel brake cylinder of the wheels 11 to 14 is connected to each outlet passage 21 to 24. Each outlet passage 21 to 24 is associated with a respective control valve 31 to 34, which is a three-port. two-position solenoid valve with spring retutn. The control valves 31 to 34 are controlled by an electronic control (not shown) and set a brake pressure in the associated wheel brake cylinders 10, which is dependent upon wheel slip. A return pump 27, which is a part of the four-path hydraulic assemblage 20, has two pump elements 28,29, which are driven jointly by an electric motor 35. The pump elements 28,29 serve to return brake fluid on - pressure reduction in the brakes. A respective one of the pump elements 28,29 is effective in each brake circuit. and on the inlet side is connected to the control valves 33,34 or 31,32 associated with this brake circuit, and, on the outlet side, to an inlet passage 25 or 26 of the four-path hydraulic assemblage 20.. A non-return valve 52, whose flow-through 1 direction is towards the pump element 29, is inserted in the connections of the control valves 31,32, which are associated with the wheel brake cylinders 10 of -8the drive wheels 11,12, to the pump element 29. A pump inlet valve 36 and a pump outlet valve 37 are disposed respectively upstream and downstream of each pump element 28,29. Low-pressure accumulator chambers 38,39, which are connected upstream of the pump inlet valves 36, permit a pressure reduction which is independent of the pump delivery quantity, and overcome the opening pressure of the pump inlet valves 36. Damping chambers 30 are also disposed in the connections between the pump outlet valves 37 and the associated inlet passages 25,26 of the four-path hydraulic assemblage 20. The inlet passages 25,26 of the four-path hydraulic assemblage 20 are connected by way of connection lines 41,42 to the brake circuit outlets 16,17 of the master brake cylinder 15.

The control valves 31 to 34 are connected in respective pairs by their first operating ports to the inlet passage 25 or 26 of the four-path hydraulic aggregate 20, by their second operating ports to the associated outlet passage 21 to 24, and by their third operating ports in respective pairs by way of the accumulator chambers 38 or 39 and the pump inlet valves 36 to the pump elements 28,29 of the return pump 27. The control valves 31 to 34 are formed in a known way such that, in their first, nonenergised basic valve position, there is unimpeded flow from the -9inlet passages 25,26 to the outlet passages 21 to 24, as a result of which the brake pressure delivered by the master brake cylinder 15 passes into the wheel brake cylinders of the wheels 11 to 14. In the second, middle valve position, which is brought about by energisation of the control valves 31 to 34 with half maximum current, this passage is interrupted and all the operating ports are closed off, such that the brake pressure which has built up in the respective wheel brake cylinders 10 is kept constant. In the third, final valve position, which is brought about by full energisation with maximum current, the outlet passages 21 and 22 or 23 and 24, and hence the wheel brake cylinders of the wheels 11 and 12 or 13 and 14, are connected to the inlet of the pump element 28 or 29, such that, for the purpose of brake pressure reduction, brake fluid can flow out of the respective wheel brake cylinders 10 into the accumulator chamber 38 or 39, and is returned by the return pump 27 into the master brake cylinder 15.

An additional hydraulic assemblage or aggregate 40 serves to provide a brake supply pressure in the event of drive slip control (ASR operation). It has a charging valve 43 (Fig.2), which is integrated in the master brake cylinder 15, a change-over valve 44, a low-pressure accumulator 45 and a switching valve 46.

_10The charging valve 43 is connected in an intake line 47, which leads by way of the master brake cylinder 15, from the brake fluid reservoir 18 to the pump inlet valve 36 of the self-priming pump element 29 of the return pump 27, and is formed in such a way that this intake line 47 is closed off in the event of brake pedal actuation. The change-over valve 44 is a three-port, two-position solenoid valve with spring return and is connected in the connection line 42 between the brake circuit outlet 17 of the master brake cylinder 15 and the inlet passage 26 of the four-path hydraulic assemblage 20. The first valve port of the changeover valve 44 is connected to the inlet passage 26, its second valve port to the brake circuit outlet 17 and its third valve port by way of a pressure-limiting valve 48 to the intake line 47. In the non-energised basic valve position, the first valve port is connected to the second valve port, and, in the valve switchrover position, to the third valve port. The switching valve 46 is a three-port, threeposition solenoid valve with spring return, and, in its non-energised basic valve position, bypasses the pressure-limiting valve 48, in its middle position, which can be brought about by energisation with half maximum current, closes this bypass of the pressurelimiting valve 48, and, in its final valve position, -11which can be brought about by energisation with maximum current, connects the low-pressure accumulator 45 to the change-over valve 44. The low-pressure accumulator 45 has, in a known manner, an accumulator cylinder 49, in which an accumulator piston 50 is axially displaceably guided, wherein the accumulator piston 50 is biassed by an accumulator spring 51.

The master brake cylinder 15 (Fig.2) has two cylinder chambers 53,54, which are each provided with a respective brake circuit outlet 16,17, and an intermediate cylinder chamber 55, which is provided. with a third outlet 56, to which the intake line 47 is connected. Each cylinder chamber 53 to 55 is connected by way of a snifter bore 57, 58 59 to the brake fluid reservoir 18, or rather to a respective one of three separate basins 60 to 62 thereof. A tandem piston 63, which is coupled by way of a connection member 64 to the brake pedal 19, has two axially displaceable brake pistons 65.66, each of which is axially displaceable in a cylinder chamber 53 or 54 against a return spring 67 or 68. An inlet valve 69 or 70 is integrated in each brake piston 65 or 66 and. after the brake piston 65 or 66 has covered a minimum displacement path, closes, and shuts off the cylinder chamber 53 or 54 from the brake fluid reservoir 18. A control spool 71, which on the one -12hand abuts against an abutment 72 for the return spring 68 of the brake piston 66, which abutment defines the cylinder chamber 54, and on the other hand, abuts against the brake piston 65 and transmits a displacement movement of the abutment 72 to the brake piston 65, is axially displaceably guided in the middle cylinder chamber 55. The control spool 71 has a control surface 711 by which it so controls a control bore 73, which is connected to the third outlet 56, that, in the event of displacement of the tandem piston 73 brought about by actuation of the brake pedal, the bore 73 is closed after a short displacement travel of the control spool 71. This displacement travel is dimensioned in such a way that the brake pressure generated in the cylinder chambers 53,54 by such short travel is still below 1 bar. The control spool 71 does not open the control bore 73 again until the brake pedal 19 is released and the return springs 67,68 have moved the tandem piston 63 back into its basic position. As long as thecontrol spool 71 opens the control bore 73, there is a direct connection between the basin 62 of the brake fluid reservoir 18 and the intake line 47 to the pump element 29 of the return pump 27. When the control bore 73 is closed, this connection is closed off, such that the pump-element 29 cannot suck in any brake -13fluid from the brake fluid reservoir 18. Cylinder chamber 55, control spool 71 and control bore 73 together form the pressure-controlled charging valve 43, which is controlled by the brake pressure being built up in the cylinder chamber 54.

The change-over valve 44 and the switching valve 46 are controlled by the electronic control (not shown). When the electronic control is informed of drive slip at at least one of the drive wheels 11,12 by wheel slip sensors (not shown), the change-over valve 44 is switched over and the switching valve 46 is moved into its third, final valve position by energisation with maximum current. At the same time, the return pump 27 is switched on. The connection line 42 is separated from the brake circuit outlet 17 of the master brake cylinder 15 and connected to the low-pressure accumulator 45 by the change-over valve 44 and the switching valve 46. In the event of little pressure, the previously charged low- pressure accumulator 45 feeds its brake fluid volume by way of the control valves 31,32 into the wheel brake cylinders 10 of the drive wheels 11,12, such that the large volume requirement of the wheel brake cylinders 10 for applying the brake calipers is very rapidly met. The pump element 29 of the starting return pump 27 sucks brake fluid out of the brake fluid reservoir -1418 by way of the intake line 47 and the opened charging valve 43 and generates a high brake pressure, which is available at the control valves 31,32 and passes by way of these valves into the wheel brake cylinders 10 of the respective slipping drive wheels 11,12. As soon as the low- pressure accumulator 45 has been substantially emptied, that is its accumulator pressure begins to fall below the brake pressure being delivered to the wheel brake cylinders 10, the switching valve 46 is switched back into its middle position, as a result of which the low- pressure accumulator 45 is switched off and the connection line 42 is connected by way of the pressure-limiting valve 48 to the intake line 47, and thus by way of the charging valve 43 to the brake fluid reservoir 18. The pressur e-limiting valve 48 thereby acts as a safety valve for the pump element 29. Switching off the low-pressure accumulator 45 prevents the lowpressure accumulator 45 being re-charged during further ASR function, and hence brake fluid volume being diverted from the brake circuit of the drive wheels 11,12.

If, for example, only the drive wheel 11 slips, the control valve 32 of the non-slipping drive wheel 12 is moved into the middle valve position, such that the outlet passage 22 is isolated from the high brake _15pressure generated by the pump element 29. By way of the other control valve 31, brake pressure is built up in the wheel brake cylinder 10 of the slipping drive wheel 11, which is braked by it. The necessary brake pressure is adjusted by pressure modulation, which is brought about by switching the control valve 31. Excess brake fluid is delivered by way of the changeover valve 44 and the pressure-limiting valve 48 back to the brake fluid reservoir 18. As soon as no more brake fluid volume is required in the brake circuit, the switching valve 46 is once again moved into its third, final valve position, as a result of which excess brake fluid refills the low-pressure accumulator 45. Brake fluid volume which is not required for this flows off by way of the pressurelimiting valve 48 to the brake fluid reservoir 18.

Towards the end of the drive slip control, when no further drive slip is sensed, the electronic control switches the control valve 31 over into its final valve position, which can be brought about by maximum energisation. In this position of the control valve 31, brake fluid flows out of the wheel brake cylinder 10 of the previously slipping drive wheel 11 into the accumulator chamber 39, and is delivered by the pump element 29 back from here by way of the change-over valve 44 into the low- pressure accumulator -1645, or by way of the pressure-limiting valve 48 into the brake fluidreservoir 18. For complete emptying of the low-pressure accumulator 39, the switching valve 46 is de-energised, as a result of which the lowpressure accumulator 45 is isolated and the pressure-limiting valve 48 is bypassed. The accumulator chamber 39 is emptied with the exception of a small residual pressure. Subsequently, the control valve 31 and the change-over valve 44 are switched back into their basic valve positions. In the event of drive slip of both drive wheels 11,12, the two control valves 31,32 are switched backwards and forwards between their basic valve position and their middle valve positions for brake pressure modulation.

In the event of brake pedal actuation, the brake piston 66 of the tandem piston 63 in Fig.2 is displaced to the left by way of the connection member 64, as a result of which the inlet valve 70 is closed. The brake pressure building up in the cylinder chamber 54 displaces the control spool 71 of the charging valve 43 by way of the abutment 72, which closes the control bore 73. Together with the control spool 71, the brake piston 65 is also displaced, as a result of which the inlet valve 69 is closed and the same brake pressure is generated in the cylinder chamber 53 as in -17the cylinder chamber 54. With closure of the control bore 73 by the control spool 71, the intake line 47 is closed off from the brake fluid reservoir 18. An equally large brake pressure is generated in the two cylinder chambers 53,54 and is delivered respectively by way of the brake circuit outlets 16.17. the connection lines 41,42 and the inlet passages 25,26 of the four-passage hydraulic assemblage 20 into the two brake circuits. A corresponding brake pressure is built up in the wheel brake cylinders 10 of the motor vehicle wheels 11 to 14 by way of the control valves. 31 to 34, which are in their basic positions. In the event of slip of one of the vehicle wheels 11 to 14, the associated control valve 31 to 34 is triggered in a known manner, and the braking system operates in ABS operation.

In the hydraulic dual-circuit braking system shown in Fig.3 with diagonal brake circuit configuration, anti-skid system (ABS) and drive slip control (ASR) for passenger vehicles, one drive wheel 11,12 is disposed in each of two brake circuits. Generally, these drive wheels 11,12 are the front wheels of the passenger vehicle. Insofar as the dualcircuit braking system coincides with the dual-circuit braking system with front/rear brake circuit configuration described in Fig.l. the same components -18have been given the same reference numerals. In the four-path hydraulic assemblage 20, the two inlet passages 25,26 are divided into two passages 25 and 251 and 26 and 26'. The outlets of the pump elements 28,29 are again connected to the inlet passages 25,26. The control valves 33 and 32, which are associated with the drive wheels 11,12 in each brake circuit, are again connected to the inlet passages 25.26. The control valves 34 and 31, which are associated with the non-driven wheels 14,13, are connected at the inlet side to the inlet passages 251 and 2C. The inlet passages 251 and 261 are connected by a third and fourth connection line 411 and 421 to the brake circuit outlets 16,17 of the master brake cylinder 15. A non-return valve 52, with a flow-through direction towards the pump element 28, is disposed in the connection between the inlet valve 36 of the pump element 28 and the control valves 33,34. The additional hydraulic assemblage 40 also has a second change-over valve 441, which is identical to the change-over valve 44. This change-over valve 441 is inserted in the first connection line 41 between the brake circuit outlet 16 of the master brake cylinder 15 and the inlet passage 25 of the four-path hydraulic assemblage 20 in such a way that its first valve port is connected to the inlet passage 25, its second valve _19port to the brake circuit outlet 16 and its third valve port to the third valve port of the change-over valve 44. The change-over valve 441 is also triggered by the electronic control as is the change-over valve 44.

The method of operation of the dual-circuit braking system of Fig.3 is identical to that of the dual-circuit braking system described with respect to Fig.l. with the difference that the two pump elements 28.29 of the return pump 27 are self-priming, and, in the event of drive slip control, the pump element 29 is used for brake pressure supply to the wheel brake cylinders 10 of the drive wheels 11,12. The lowpressure accumulator 45 in turn ensures rapid filling of the wheel brake cylinders 10 of the slipping drive wheels 11,12 on commencement of drive-slip control in the event of a low pressure level.

The invention is not restricted to the embodiments described. Each control valve may be formed from a combination of two two-port, twoposition solenoid valves. A two-port,, two-position solenoid valve forms an inlet valvet which, in its unenergised basic position, permits unimpeded passage from the inlet passage to the associated outlet passage, and, in its operated position, closes off this passage. Conversely, the other two-port, twoposition solenoid valve, which acts as a so-called -20outlet valve, forms, in its operated position, which can be brought about by magnetic energisation, a connection between the associated outlet passage and the inlet of the associated pump element, and, in its non-energised basic position, closes off this passage. In the so-called pressure-holding position, the two two-port, two-position solenoid valves of the valve combination both take up their closed position.

In the case of long lines between the brake fluid reservoir and the return pump, it is expedient to insert in the intake line a pre-charging pump, which is switched on in the event of drive slip control.

In Fig.4, the same master brake cylinder 10 is shown in part as in Fig.2, in which, however, the integrated charging valve 43 is in the form of a tilting valve 80. The tilting valve 80 has a valve member 81, which is screwed into the housing of the master brake cylinder 15 and which has a valve opening S2 surrounded by a valve seat 83 and continuing into the third outlet 56 of the master brake cylinder 15. Under the effect of a trapezoidal spring 84, a plateshaped valve member 85 is seated on the valve seat 83 and carries an actuating finger 86, which is coaxially surrounded by the trapezoidal spring 84. Valve member 85 and trapezoidal spring 84 lie in a bore 87 in the housing of the master cylinder 15, which bore is -21closed off by the valve member 81 and is connected by way of a reduced- diameter connection bore 88 to the middle cylinder chamber 55. In the middle cylinder chamber 55, there is inserted a spacer piston 89, which, by means of two piston lands 91,92, which are disposed at a distance from one another, is axially displaceably guided in the cylinder chamber 55, and is sealed with respect to the wall of the cylinder chamber 55 by means of annular seals 90 in these lands. The spacer piston 89 abuts on the one hand against the abutment 72 for the return spring 68 of the brake piston 66 and, on the other hand, against the brake piston 65, and transmits the displacement movement of the brake piston 66 to the brake piston 65. In any displaced position of the spacer piston 89, the connection bore 88 lies between the two piston lands 91,92. The actuating finger 86 of the tilting valve 80 projects through the connection bore 88 into the cylinder chamber 55, wherein the tilting valve 80 and the connection bore 88 are disposed relative to the tandem piston 63 in such a way that, in its basic position, the actuating finger 86 abuts against a radial shoulder 93 of the spacer piston 89, and is thus deflected from the valve axis, such that the valve member 85 is tilted with respect to the valve seat 83, and the valve opening 82 is freed. As soon -22as the tandem piston 63 has moved out of its basic position as a result of brake pedal actuation (to the left in Fig.4), the radial shoulder 93 of the spacer piston 89 releases the actuating finger 86, and the valve member 85 is pressed by the trapezoidal spring 84 onto the valve seat 83.

The charging valve 43, which is integrated in the master brake cylinder 15, may also be in the form of a ball-seat valve, which is kept open, for example, by way of a tappet in the basic position of the tandem piston 63. One such seat valve is described, for example, in DE 32 36 581 Al, such that it does not need to be described further here.

Claims

-23CIAIMS

1. A hydraulic braking system having an antiskid system and drive slip control for a motor vehicle, comprising a master brake cylinder, having at least one brake circuit outlet, for delivering a brake pressure upon actuation of a brake pedal, a brake fluid reservoir, which is connected to the master brake cylinder, and a hydraulic assemblage, which is connected to the at least one brake circuit outlet of the master brake cylinder and is disposed in advance of at least one wheel brake cylinder of a motor vehicle wheel, and which comprises a charging valve and at least one changeover valve and, by means of the charging valve, at least in the event of drive slip control, connects at least one pump element of a return pump, which is associated with a brake circuit having.at least one drive wheel and is self-priming, to-the brake fluid reservoir, and, at least in the event of brake pedal actuation, interrupts this connection, and, by means of the at least one changeover valve which, in the event of drive-slip control, isolates the at least one pump element, which belongs to a brake circuit having at least one drive wheel, from the master brake cylinder, and connects it by way of a pressure-l-im- iting valve to the brake fluid reservoir, in which the charging valve is integrated -24in the master brake cylinder and has a valve opening which is controlled by a valve element and which lies between the brake fluid reservoir and a further outlet of the master brake cylinder, and the valve element is coupled to the brake pedal, and in which an intake line leads to the at least one pump element of the return pump, which is associated with a brake circuit having at least one drive wheel and is self-priming, and is connected to the further outlet of the master brake cylinder.

2. A braking system as claimed in claim 1, in which the master brake cylinder has at least one cylinder chamber, which is provided with a brake circuit outlet and which is connected by way of a snifter bore to the brake fluid reservoir, and at least one brake piston, which is coupled to the brake pedal and is axially displaceable in the cylinder chamber, and which covers the snifter bore upon axial displacement and puts the brake fluid in the cylinder chamber under pressure, and in which the charging valve comprises a spool valve, whose control spool is coupled to the at least one brake piston and is axially displaceably inserted in a further cylinder chamber of the master brake cylinder, the further cylinder chamber is connected on the one hand by way of a control bore, which forms the valve opening, to -25the further outlet of the master brake cylinder, and, on the other hand, by way of a further snifter bore to the brake fluid reservoir, and the control bore is disposed such that it is closed by the control spool in the event of axial displacement of the brake piston caused by brake pedal actuation.

3. A braking system as claimed in claim 1, in which the master brake cylinder has at least one cylinder chamber, which is provided with a brake circuit outlet and which is connected by way of a snifter bore to the brake fluid reservoir, and at least one brake piston, which is coupled to the brake pedal and is axially displaceable in the cylinder chamber, and which covers the snifter bore on axial displacement and puts the brake fluid in the cylinder chamber under pressure, and in which the charging valve comprises a seat valve, whose valve seat surrounds the valve opening and whose valve member, which co-operates with the valve seat, is coupled to the at least one brake piston in such a way that. in the basic position of the brake piston, the valve member is raised by the brake piston from the valve seat, and, in the event of an axial displacement. which is caused by brake pedal actuation, is released for a closing movement caused by a closure spring.

v

4. A braking system as claimed in claim 3, in which the seat valve is a tilting valve, whose valve member projects into the displacement path of the brake piston and is disposed relative to the brake piston in such a way that, in the basic position of the brake piston, the valve member is tilted by the latter against the return force of a trapezoidal spring and thus releases the valve opening, and, in the event of an axial displacement caused by brake pedal actuation, is released by the brake piston for return.

S. A braking system as claimed in claim 4, in which the tilting valve has a valve member, which is inserted in the housing of the master brake cylinder and which bears the valve opening with valve seat and is connected by way of a housing bore to a further cylinder chamber, which is defined by the brake piston, the further cylinder chamber is permanently connected by way of a snifter bore to the brake fluid reservoir, and the valve member projects by an actuating finger through the housing bore into the further cylinder chamber in such a way that, in the basic position of the brake piston, the actuating finger is deflected from the valve axis.

6. A braking system as claimed in any of claims 1 to 5, in which the hydraulic assemblage includes a t v - 27pressure accumulator, which can be connected by way of an electromagnetic switching valve to a line portion, which lies between the change-over valve and the pressure-limiting valve, and which is a portion of a connection line to the pump element of the return pump, which is associated with a brake circuit having at least one drive wheel, from the associated brake circuit outlet of the master brake cylinder.

7. A braking system as claimed in claim 6, in which the switching valve comprises a three-port, three-position solenoid valve with spring return, which, in one valve position, closes off the lowpressure accumulator and bridges the pressure-limiting valve, in its second valve position, closes off the low-pressure accumulator and interrupts the bypass, and in its third valve position, connects the lowpressure accumulator to the line portion between the change-over valve and the pressure-limiting valve.

8. A braking system constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

Published 1991 atlbe Patent Office, Concept House, Cardiff Road. Newport. Gwent NP9 I RH- Further copies may be obtained from rwmfeltnfach. Cross Keys, Newport. NPI 7HZ. Printed by Multiplex techniques lid, St Mary Cray, Kent.

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