DE4016560A1 - HYDRAULIC BRAKE SYSTEM - Google Patents

Description

State of the art

The invention is based on a hydraulic Brake system with anti-lock system (ABS) and Traction control system (ASR) for motor vehicles in the Preamble of claim 1 defined genus.

In a known brake system of this type (DE 38 16 073 A1) with a front / rear axle Brake circuit division is the charging valve as a pressure-controlled 2/2-way valve designed, the hydraulic control input on a brake circuit output of the Master brake cylinder is connected. The 2/2-way valve is a separate valve in the feed line between the Brake fluid reservoir and the entrance of the brake circuit the pump element associated with the drive wheels switched on. When the brake pedal is actuated, the charging valve is reversed and shuts off the feed line, so that none during ABS operation  and can be promoted into the brake circuit.

Advantages of the invention

The brake system according to the invention characterizing features of claim 1 has the advantage that a separately assembled, pressure-controlled valve with The locking function when the brake pedal is actuated is omitted. Through the Integration in the master brake cylinder can also Function of the loading valve has been significantly improved than that the valve opening cross sections of the advantageously designed as a slide valve loading valve can be made larger and closing the valve even with the brake pressure controlled in the master brake cylinder of less than 1 bar occurs.

By the measures listed in the other claims are advantageous developments and improvements in Claim 1 specified brake system possible.

The hydraulic unit for brake pressure supply to the Wheel brake cylinder can while maintaining the im Master brake cylinder integrated charging valve versatile be designed. For large-volume wheel brake cylinders that large brake fluid volumes for applying the brake calipers need, it is advantageous if according to a Embodiment of the invention a low pressure accumulator is provided, which at the beginning of ASR operation Changeover valve and control valves of the hatching Drive wheels feeds directly into their wheel brake cylinder. As soon as the pressure of the low pressure accumulator is less than the brake supply pressure generated by the return pump switches between the low pressure accumulator and the Switching valve arranged electromagnetic  Switch valve in the blocking position and disconnects the Low pressure accumulator from the brake supply circuit. So that will prevents the low pressure accumulator from already during the Brake pressure build-up in the wheel brake cylinders brake fluid withdraws from the brake supply circuit for its filling. As soon as the brake fluid requirement of the wheel brake cylinder is covered and a sufficiently high brake pressure in the Wheel brake cylinders has been built that switches Switch valve again so that the memory again with the Brake supply circuit is connected and with excess Brake fluid can be charged. About that brake fluid volumes going beyond that Pressure relief valve to the master brake cylinder promoted back and get out of it with the third Output of the master cylinder connected cylinder space in the brake fluid reservoir.

According to a further embodiment of the invention, this is Switch valve between low pressure accumulator and changeover valve designed as a 3/3-way solenoid valve with spring return, in its one valve position the low-pressure accumulator shuts off and bridges the pressure relief valve, in its second valve position the low pressure accumulator isolates and bridges the pressure relief valve picks up and in its third valve position Low pressure accumulator connects to the changeover valve. These Training of the switching valve has the advantage that at Brake pressure reduction in ASR operation the pressure relief valve can be bridged and thus low pressures in the the low-pressure storage chambers connected to the pump elements can be achieved as a result of which an essential faster transition to ABS operation is possible.  

drawing

The invention is illustrated in the drawing Exemplary embodiments in the description below explained in more detail. Show it:

Fig. 1 is a block diagram of a dual-circuit brake system with front / rear axle brake circuit division, anti-lock system and traction slip control for a passenger car,

Fig. 2 is a detail in longitudinal section of a brake master cylinder of the Dual circuit in Fig. 1, shown schematically,

Fig. 3 is a block diagram of a dual-circuit brake system with diagonal brake circuit allotment,

Fig. 4 shows the same representation as in Fig. 2 according to another embodiment.

Description of the embodiments

In the hydraulic dual-circuit brake system shown in FIG. 1 with front / rear or front axle / rear axle or so-called black / white brake circuit division, anti-lock braking system (ABS) and traction control (ASR), also called propulsion control, the wheel brake cylinders are for a passenger car 10 of the drive wheels 11 , 12 in one brake circuit and the wheel brake cylinders 10 of the non-driven wheels 13 , 14 in the other brake circuit. In general, the drive wheels 11 , 12 are the rear wheels of the passenger car. The dual-circuit brake system includes, in a manner known per se, a master brake cylinder 15 which has two separate brake circuit outputs 16 , 17 for connecting one of the two brake circuits in each case and is connected to a brake fluid reservoir 18 . When a brake pedal 19 is actuated, an equal brake pressure is controlled in the brake circuits via the two brake circuit outputs 16 , 17 .

The dual-circuit brake system also includes a four-channel hydraulic unit 20 which has four outlet channels 21-24 and two inlet channels 25 and 26 . A wheel brake cylinder 10 of the wheels 11-14 is connected to each outlet channel 21-24 . A control valve 31-34 designed as a 3/2-way solenoid valve with spring return is assigned to each outlet channel 21-24 . The control valves 31-34 are controlled by control electronics (not shown here) and set a brake pressure dependent on the wheel slip in the assigned wheel brake cylinders 10 . A return pump 27 , which is part of the four-channel hydraulic unit 20 , has two pump elements 28 , 29 , which are driven together by an electric motor 35 . The pump elements 28 , 29 serve to return brake fluid when the pressure in the brakes is reduced. One pump element 28 , 29 is effective in a brake circuit and is connected on the input side to the control valves 33 , 34 and 31 , 32 assigned to this brake circuit and on the output side to an inlet channel 25 and 26 of the four-channel hydraulic unit 20 . In the connections of the control valves 31 , 32 assigned to the wheel brake cylinders 10 of the drive wheels 11 , 12 to the pump element 29 , a check valve 52 with a flow direction directed towards the pump element 29 is switched on. A pump inlet valve 36 and a pump outlet valve 37 are arranged before and after each pump element 28 , 29 . Low-pressure storage chambers 38 , 39 connected upstream of the pump inlet valves 36 allow a pressure reduction regardless of the pump delivery rate and overcome the opening pressure of the pump inlet valves 36 . Damping chambers 30 are also arranged in the connections between the pump outlet valves 37 and the assigned inlet channels 25 , 26 of the four-channel hydraulic unit 20 . The inlet channels 25 , 26 of the four-channel hydraulic unit 20 are connected to the brake circuit outputs 16 , 17 of the master brake cylinder 15 via connecting lines 41 , 42 .

The control valves 31-34 are in pairs with their first working connection to an inlet channel 25 or 26 of the four-channel hydraulic unit 20 , with their second working connection to the assigned outlet channel 21-24 and with their third working connection in pairs via the storage chamber 38 or 39 and the pump inlet valves 36 connected to the pump elements 28 , 29 of the return pump 27 . The control valves 31-34 are designed in a known manner so that in their first, non-energized valve basic position there is an unhindered passage from the inlet channels 25 , 26 to the outlet channels 21-24 , as a result of which the brake pressure controlled by the master cylinder 15 into the wheel brake cylinders 10 of the wheels 11-14 arrives. In the second valve center position, which is brought about by excitation of the control valves 31-34 with half the maximum current, this passage is interrupted and all work connections are shut off, so that the brake pressure built up in the wheel brake cylinders 10 is kept constant. In the third valve end position, which is set by valve excitation with maximum current, the output channels 21 and 22 or 23 and 24 and thus the wheel brake cylinders 10 of the wheels 11-14 are connected to the input of the pump elements 28 and 29 , so that brake fluid is used for brake pressure reduction can flow out of the wheel brake cylinders 10 into the storage chambers 38 , 39 and is returned by the return pump 27 into the master brake cylinder 15 .

An additional hydraulic unit 40 serves to provide a brake supply pressure in the case of traction control (ASR operation). It has a charging valve 43 ( FIG. 2) integrated in the master brake cylinder 15 , a changeover valve 44 , a low pressure accumulator 45 and a switching valve 46 . The charging valve 43 is switched on in a suction line 47 guided via 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 designed such that this suction line 47 is blocked when the brake pedal is actuated. The changeover valve 44 is designed as a 3/2-way solenoid valve with spring return and is switched on in the connecting line 42 between the brake circuit outlet 17 of the master brake cylinder 15 and the inlet channel 26 of the four-channel hydraulic unit 20 . Its first valve connection is connected to the inlet channel 26 , its second valve connection to the brake circuit outlet 17 and its third valve connection to the suction line 47 via a pressure relief valve 48 . The first valve connection is connected to the second valve connection in the de-energized basic valve position and to the third valve connection in the valve switching position. The switching valve 46 is designed as a 3/3-way solenoid valve with spring return which bridges the pressure relief valve 48 in its non-energized basic valve position, in its middle position which can be brought about by excitation with half the maximum current, cancels the short circuit of the pressure relief valve 48 and in its valve end position which can be brought about by excitation with maximum current the low pressure accumulator 45 connects to the switching valve 44 . The low-pressure accumulator 45 has, in a known manner, a storage cylinder 49 in which a storage piston 50 is guided in an axially displaceable manner, the storage piston 50 being loaded by a storage spring 51 .

The master brake cylinder 15 is shown in FIG. 2 in detail and schematically in longitudinal section. It has two cylinder spaces 53 , 54 each provided with a brake circuit output 16 , 17 and a cylinder space 55 lying in between, which is provided with a third output 56 to which the suction line 47 is connected. Each cylinder space 53-55 is connected to the brake fluid reservoir 18 via a sniffing bore 57 , 58 , 59 , in each case with one of three basins 60-62 which are separated from one another. A tandem piston 63 , which is coupled via a connec tion member 64 with the brake pedal 19 , has two axially displaceable brake pistons 65 , 66 , one of which is axially displaceable against a return spring 67 or 68 in a cylinder chamber 53 or 54 . In each brake piston 65 and 66 , an inlet valve 69 or 70 is inte grated, which closes after covering a minimal displacement of the brake piston 65 or 66 and blocks the cylinder space 53 or 54 from the brake fluid reservoir 18 . In the middle cylinder space 55 , a control slide 71 is axially displaceably guided, which on the one hand rests on an abutment 72 for the return spring 68 of the brake piston 66 delimiting the cylinder space 54 and on the other hand on the brake piston 65 and transmits a displacement movement of the abutment 72 to the brake piston 65 . The spool valve 71 controls with a control surface 711 a standing to the third output 56 in connection control bore 73 such that these in-induced displacement of the brake pedal operating tandem piston 63 by a small displacement of the control slide is closed 71st This displacement path is dimensioned such that the brake pressure generated in the cylinder spaces 53 , 54 is still below 1 bar. The control slide 71 only opens the control bore 73 again when the brake pedal 19 is released and the return springs 67 , 68 have reset the tandem piston 63 into its basic position. As long as the control slide 71 releases the control bore 73 , there is a direct connection between the basin 62 of the brake fluid reservoir 18 and the suction line 47 to the pump element 29 of the return pump 27 . When the control bore 73 is closed, this connection is blocked, so that the pump element 29 can not suck in brake fluid from the brake fluid reservoir 18 . Cylinder chamber 55 , control slide 71 and control bore 73 together form the pressure-controlled charging valve 43 , which is controlled by the brake pressure built up in the cylinder chamber 54 .

The switching valve 44 and the switching valve 46 are controlled by the control electronics, not shown. If the control electronics are informed of at least one of the drive wheels 11, 12 by wheel slip sensors (not shown here), the changeover valve 44 is switched over and the switching valve 46 is moved into its third valve end position by maximum current excitation. At the same time, the return pump 27 is switched on. The changeover valve 44 and switching valve 46 separate the connecting line 42 from the brake circuit outlet 17 of the master brake cylinder 15 and connect them to the low-pressure accumulator 45 . The tensioned low-pressure accumulator 45 feeds its brake fluid volume via the control valves 31 , 32 into the wheel brake cylinders 10 of the drive wheels 11 , 12 at low pressure, so that the large volume requirement of the wheel brake cylinders 10 for applying the brake calipers is covered very quickly. The pump element 29 of the starting return pump 27 sucks brake fluid from the brake fluid reservoir 18 via the suction line 47 and the opened charging valve 43 and generates a high brake pressure which is applied to the control valves 31 , 32 and into the wheel brake cylinders 10 of the respective slipping drive wheels 11 , 12 arrives. As soon as the low-pressure accumulator 45 is largely emptied, that is, its accumulator pressure begins to fall below the brake pressure which is controlled in the wheel brake cylinders 10 , the switching valve 46 switches back to its central position, as a result of which the low-pressure accumulator 45 is switched off and the connecting line 42 via the pressure-limiting valve 48 to the suction line 47 and is connected to the brake fluid container 18 via the charging valve 43 . By switching off the low-pressure accumulator 45 , it is prevented that the low-pressure accumulator 45 is tensioned again during the further ASR function and thus brake fluid volume is skimmed out of the brake circuit of the drive wheels 11 , 12 .

If, for example, only the drive wheel 11 rotates, the control valve 32 of the non-slipping drive wheel 12 is moved into the valve center position, so that the outlet channel 22 is shut off from the high brake pressure generated by the pump element 29 . Brake pressure is built up in the wheel brake cylinder 10 of the spinning drive wheel 11 via the other control valve 31 , and is thus braked. The required brake pressure is set by pressure modulation, which is brought about by switching the control valve 31 . Excess brake fluid is returned to the brake fluid reservoir 18 via the changeover valve 44 and the pressure relief valve 48 . As soon as no more brake fluid volume is required in the brake circuit, the switching valve 46 is moved back into its third valve end position, as a result of which excess brake fluid fills the low-pressure accumulator 45 again. Brake fluid volume not required for this flows through the pressure limiting valve 48 to the brake fluid reservoir 18 .

Towards the end of the traction control, when no more traction is sensed, the control electronics switch the control valve 31 into its valve end position that can be brought about by maximum current excitation. With this position of the control valve 31 , brake fluid flows out of the wheel brake cylinder 10 of the drive wheel 11 into the storage chamber 39 and is conveyed back from here by the pump element 29 via the changeover valve 44 into the low-pressure accumulator 45 or via the pressure-limiting valve 48 into the brake fluid reservoir 18 . To completely empty the low-pressure accumulator 39 , the switching valve 46 is de-energized, as a result of which the low-pressure accumulator 45 is separated and the pressure-limiting valve 48 is bridged. The storage chamber 39 is emptied to a small residual pressure. The control valve 31 and the changeover valve 44 are then switched back into their basic valve position. When both drive wheels 11 , 12 slip, both control valves 31 , 32 are switched back and forth for brake pressure modulation between the basic valve position and the central valve position.

When the brake pedal is actuated, the brake piston 66 of the tandem piston 63 is displaced to the left in FIG. 2 via the connecting member 64 , as a result of which the inlet valve 70 is closed. The brake pressure building up in the cylinder space 54 displaces the control slide 71 of the loading valve 43 via the abutment 72 , which closes the control bore 73 . Together with the control slide 71 , the brake piston 65 is also displaced, as a result of which the inlet valve 69 closes and the same brake pressure is generated in the cylinder space 53 as in the cylinder space 54 . When the control bore 73 is closed by the control slide 71 , the suction line 47 is shut off from the brake fluid reservoir 18 . In the two cylinder chambers 53 , 54 , an equal brake pressure is controlled, which reaches the two brake circuits via the brake circuit outputs 16 , 17 , the connecting lines 41 , 42 and the inlet channels 25 , 26 of the four-channel hydraulic unit 20 . A corresponding brake pressure is built up in the wheel brake cylinders 10 of the vehicle wheels 11-14 via the control valves 31-34 in their basic position. If one of the vehicle wheels 11-14 slips , the associated control valve 31-34 is actuated in a known manner and the brake system operates in ABS mode.

In the hydraulic dual-circuit brake system shown in FIG. 3 with a diagonal brake circuit division, anti-lock system (ABS) and traction control system (ASR) for passenger cars, a drive wheel 11 , 12 is arranged in one of two brake circuits. In general, these drive wheels 11 , 12 are the front wheels of the passenger car. Insofar as the dual-circuit brake system corresponds to the dual-circuit brake system with front / rear brake circuit division described in FIG. 1, the same components are provided with the same reference symbols. In the four-channel hydraulic unit 20 , the two inlet channels 25 , 26 are each separated into two channels 25 and 25 'or 26 and 26 '. The outlets of the pump elements 28 , 29 are still connected to the inlet channels 25 , 26 . The control valves 33 and 32 , which are assigned to the drive wheels 11 , 12 in each brake circuit, are also connected to the inlet channels 25 , 26 . With the inlet channels 25 'and 26 ', the control valves 34 and 31 , which are associated with the non-driven wheels 14 , 13 , are connected on the input side. The inlet channels 25 'and 26 ' are connected by a third and fourth connecting line 41 'and 42 ' to the brake circuit outputs 16 , 17 of the master cylinder 15 . In the connection between the inlet valve 36 of the pump element 28 and the control valves 33 , 34 , a check valve 52 is arranged with the forward direction pointing to the pump element 28 . The additional hydraulic unit 40 also has a second switch valve 44 ', which is identical to the switch valve 44 . This changeover valve 44 'is switched on in the first connecting line 41 between the brake circuit output 16 of the master brake cylinder 15 and the inlet channel 25 of the four-channel hydraulic unit 20 so that its first valve connection to the inlet channel 25 , its second valve connection to the brake circuit output 16 and its third valve connection the third valve port of the changeover valve 44 . The switching valve 44 'is also controlled by the control electronics at the same time as the switching valve 44 .

The operation of the dual-circuit brake system is identical to that of the dual-circuit brake system described in FIG. 1, with the difference that now both pump elements 28 , 29 of the return pump 27 are designed to be self-priming and are used in the case of traction control to supply brake pressure to the wheel brake cylinders 10 of the drive wheels 11 , 12 . The low-pressure accumulator 45 in turn ensures that the wheel brake cylinders 10 of the slipping drive wheels 11 , 12 are filled quickly at the start of the traction control system at a low pressure level.

The invention is not based on those described Embodiments limited. So every control valve formed from a combination of two 2/2-way solenoid valves will. A 2/2-way solenoid valve forms one Inlet valve, which in its deenergized home position free passage from the inlet duct to the associated outlet channel allows and in its Working position blocks this passage. Conversely poses the other, acting as a so-called outlet valve 2/2-way solenoid valve in its by magnetic excitation bring about working position a connection of the assigned outlet channel to the input of the assigned Pump element forth and locks in its unenergized Home position this passage. In the so-called Both 2/2-way solenoid valves take the pressure holding position Valve combination their locked position.  

With long line lengths between the brake fluid reservoir and return pump it is appropriate in the suction line turn on a pre-charge pump, which at Traction control is switched on.

FIG. 4 shows the same master brake cylinder 10 as a detail in FIG. 2, but in which the integrated charging valve 43 is designed as a toggle valve 80 . The tilting valve 80 has a valve body 81 screwed into the housing of the master brake cylinder 15 , which has a valve opening 82 which continues into the third outlet 56 of the master brake cylinder 15 and is surrounded by a valve seat 83 . Under the action of a trapezoidal spring 84, a plate-shaped valve member 85 rests 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 is closed off by the valve body 81 and which communicates with the central cylinder space 55 via a reduced diameter bore 88 . In the middle cylinder chamber 55, a spacer 89 is a piston, which is guided by means of two spaced apart from one another piston slide surfaces 91 92 in the cylinder chamber 55 axially movable and sealed by means of annular seals 90 relative to the wall of the cylinder space 55th The spacer piston 89 rests on the end face on the one hand on the abutment 72 for the return spring 68 of the brake piston 66 and on the other hand on the brake piston 65 and transmits the displacement movement of the brake piston 66 to the brake piston 65 . The connecting bore 88 lies in each displacement position of the spacer piston 89 between the two piston sliding surfaces 91 , 92 . The actuating finger 86 of the tilt valve 80 protrudes through the connecting bore 88 into the cylinder space 55 , the tilt valve 80 and the connecting bore 88 being arranged relative to the tandem piston 63 so that in the basic position the actuating finger 86 rests against a radial shoulder 93 of the spacer piston 89 and thereby is deflected from the valve axis, so that the valve member 85 is tilted relative to the valve seat 83 and the valve opening 82 is opened. Once the tandem piston has moved 63 as a result of brake pedal actuation from its basic position (in Fig. 4 to the left), the radial shoulder 93 outputs the distance the piston 89, the actuating finger 86 freely, and the valve member 85 is pressed by the trapezoidal spring 84 on the valve seat 83rd

The charging valve 43 integrated in the master brake cylinder 15 can also be designed as a ball seat valve, which is kept open, for example, via a tappet in the basic position of the tandem piston 63 . Such a seat valve is described, for example, in DE 32 36 581 A1, so that it does not need to be discussed in more detail here.

Claims (7)

1.Hydraulic brake system with anti-lock system and traction control system for motor vehicles with a master brake cylinder having at least one brake circuit output for controlling a brake pressure by actuating the brake pedal, with a brake fluid reservoir connected to the master brake cylinder, with at least one wheel brake cylinder of a vehicle wheel connected to the at least one brake circuit output of the master brake cylinder Upstream hydraulic unit, which connects at least one self-priming pump element of a return pump to a brake circuit with at least one drive wheel and connects the brake fluid reservoir to the brake fluid reservoir by means of a charging valve and at least in the case of traction control and disconnects this connection at least when the brake pedal is actuated, and by means of at least one changeover valve in the case of traction control, the at least one brake circuit with at least one pump element associated with a drive wheel separates from the master brake cylinder and connects to the brake fluid reservoir via a pressure relief valve, characterized in that the charging valve ( 43 ) is integrated in the master brake cylinder ( 15 ) in such a way that a valve opening ( 73 ) of the loading valve ( 43 ) controlled by a valve element is located between the brake fluid reservoir ( 18 ) and a further output ( 56 ) of the master brake cylinder ( 15 ), that the valve element is coupled to the brake pedal ( 19 ) and that at the further output ( 56 ) of the master brake cylinder ( 15 ) one to the at least one brake circuit At least one drive wheel ( 11 , 12 ) associated with the self-priming pump element ( 29 ) of the return pump ( 27 ) is connected to the suction line ( 47 ). 2. Brake system according to claim 1, in which the master brake cylinder has at least one cylinder chamber provided with a brake circuit output, which is connected to the brake fluid reservoir via a sniffer bore, and at least one brake piston coupled to the brake pedal and axially displaceable in the cylinder chamber, which has the sniffer bore when axially displaced covers and pressurizes the brake fluid located in the cylinder space, characterized in that the charging valve ( 43 ) is designed as a slide valve, the control slide ( 71 ) of which is coupled to the at least one brake piston ( 65 , 66 ) and in a further cylinder space ( 55 ) of the master brake cylinder ( 15 ) is axially displaceable so that the further cylinder space ( 55 ) is connected on the one hand via a control bore ( 73 ) forming the valve opening to the further outlet ( 56 ) of the master brake cylinder ( 15 ) and on the other hand via a further sniffer bore ( 59 ) mi t the brake fluid container ( 18 ) is in communication, and that the control bore ( 73 ) is arranged so that it is closed by an axial displacement of the brake piston caused by the brake piston ( 65 , 66 ) by the control slide ( 71 ). 3. Brake system according to claim 1, in which the master brake cylinder has at least one cylinder chamber provided with a brake circuit output, which is connected to the brake fluid reservoir via a sniffer bore, and at least one brake piston coupled to the brake pedal and axially displaceable in the cylinder chamber, which has the sniffer bore when axially displaced covers and pressurizes the brake fluid located in the cylinder chamber, characterized in that the charging valve ( 43 ) is designed as a seat valve ( 80 ), the valve seat ( 83 ) of which surrounds the valve opening ( 82 ) and the valve member which interacts with the valve seat ( 83 ) ( 85 ) is coupled to the at least one brake piston ( 65 , 66 ) in such a way that the valve member ( 85 ) is lifted from the valve seat ( 83 ) in the basic position of the brake piston ( 65 , 66 ) and in the event of an axial displacement caused by brake pedal actuation Sc caused by a closing spring locking motion is released. 4. Brake system according to claim 3, characterized in that the seat valve is a tilt valve ( 80 ), the valve member ( 83 ) protrudes into the displacement of the brake piston ( 65 , 66 ) and is arranged relative to the brake piston ( 65 , 66 ) so that the valve member ( 83 ) in the basic position of the brake piston ( 65 , 66 ) is tilted by the latter against the return force of a trapezoidal spring ( 83 ) and thus releases the valve opening ( 82 ) and in the event of an axial displacement of the brake piston ( 65 , 66 ) caused by brake pedal actuation Provision is released. 5. Brake system according to claim 4, characterized in that the tilting valve ( 80 ) has a valve body ( 81 ) inserted into the housing of the master brake cylinder ( 15 ), which carries the valve opening ( 82 ) with valve seat ( 83 ) and via a housing bore ( 88 ) is connected to a further cylinder chamber ( 55 ) bounded by the brake piston ( 65 , 66 ), that the further cylinder chamber ( 50 ) is constantly connected to the brake fluid reservoir ( 18 ) via a sniffer bore ( 59 ) and that the valve member ( 85 ) is connected to an actuating finger ( 86 ) projects through the housing bore ( 88 ) into the further cylinder space ( 55 ) such that the actuating finger ( 86 ) is deflected out of the valve axis in the basic position of the brake piston ( 65 , 66 ). 6. Brake system according to one of claims 1-5, characterized in that the hydraulic unit ( 40 ) has a low-pressure accumulator ( 45 ), which via an electromagnetic switching valve ( 46 ) to between the switching valve ( 44 ; 44 , 44 ') and the Line section of a connecting line ( 42 ; 42 , 41 ) of the pump element ( 29 ; 29 , 28 ) assigned to a brake circuit with at least one drive wheel of the return pump ( 27 ) to the assigned brake circuit outlet ( 17 ; 17 , 16 ) of the master brake cylinder ( 52 ) 15 ) can be connected. 7. Brake system according to claim 6, characterized in that the switching valve ( 46 ) is designed as a 3/3-way magnetic valve with spring return, which shuts off the low-pressure accumulator ( 45 ) in its one valve position and bridges the pressure limiting valve ( 48 ) in its second valve position shuts off the low-pressure accumulator ( 45 ) and removes the bridging and in its third valve position connects the low-pressure accumulator ( 45 ) to the line section between changeover valve ( 44 ; 44 , 44 ') and pressure-limiting valve ( 48 ).