GB2193546A

GB2193546A - Braking-skid-controlled brake system

Info

Publication number: GB2193546A
Application number: GB08717931A
Authority: GB
Inventors: Hans-Christof Klein
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co oHG
Priority date: 1986-08-04
Filing date: 1987-07-29
Publication date: 1988-02-10
Anticipated expiration: 2007-07-29
Also published as: GB8717931D0; GB2226859B; FR2602196A1; GB2226859A; DE3626388A1; GB9001731D0; GB2193546B; FR2602196B1

Abstract

A brake system for automotive vehicles comprises a vacuum brake force booster (14) the piston wall (11) of which contains a quick-switching pneumatic short-circuiting valve (18) to be opened during wheel slip control. The short-circuiting valve (18) comprises a valve disc (20) covering a connecting port (19) between the pressure chamber (16) and the vacuum chamber (17) on the pressure chamber (16) side and openable by an electromagnet (21). The actuating plunger (22) of the valve disc (20) extends, opposite the opening direction, to a compensating piston (23) provided thereon for compensating for the opposed areas of the disc and acted upon, in the valve opening direction, by the pressure of the pressure chamber (16) and, in the closing direction, by the vacuum of the vacuum chamber (17). In the braking system of Fig. 1 the pressures in the booster chambers (16,17) are determined by a differential piston device (14') connected to a vacuum tank (48) and including a pedal actuated vacuum valve (45) which inlets atmospheric air to a chamber (16'). During skid control two valves (50) are closed and the valve (18) is selectively actuated. <IMAGE>

Description

SPECIFICATION Braking-skid-controlled brake system This invention relates to a brake system, for automotive vehicles, comprising a braking skid control and a vacuum brake force booster disposed between a brake pedal and an hydraulic master cylinder, wherein a quick-acting pneumatic short-circuiting valve to which pressure is applied by a control circuit of the braking skid control, is provided in a piston wall of the brake force booster to cause, during a braking operation and upon occurrence of a braking skid, a pressure compensation between a pressure and a vacuum chamber reducing the braking skid.

A brake system for automotive vehicles of the afore-described type, comprising a shortcircuiting valve provided in the piston wall of the vacuum brake force booster is known in the art (West German DE-OS 3428869). A special problem involved with short-circuiting valves of this type resides in that they fail to expose adequately large flow cross-sections and that the operation thereof requires substantial forces in particular in cases where they have to work against the pressure in the pressure chamber.

It is, therefore, an object of the present invention to provide a brake system for automotive vehicles of the afore-mentioned type, wherein the short-circuiting valve can be quickly opened by applying relatively low forces substantially non-influenced by the pressure difference on the piston wall to then expose a comparatively large overflow crosssection between the pressure chamber and the vacuum chamber.

According to the present invention there is provided a brake system, for automotive vehicles, comprising a braking skid control and a vacuum brake booster disposed between a brake pedal and an hydraulic master cylinder, wherein a quick-acting pneumatic short-circuiting valve to which pressure is applied by a control circuit of the braking skid control is provided in a piston wall of the booster, which short-circuiting valve, during a braking operation and upon occurrence of a braking skid, causes a pressure compensation between a pressure chamber and a vacuum chamber, reducing the braking skid, characterised in that the short-circuiting valve comprises a valve disc covering a connecting bore between the pressure chamber and the vacuum chamber from the pressure chamber side and being removable from the connecting port by means of an electromagnet in the direction of the pressure chamber, that is the opening direction, and that the actuating plunger of the valve disc in a direction opposite the opening direction extends to a compensating piston provided thereon and acted upon, from the side opposite the opening direction, by the pressure of the pressure chamber and, from the side facing the opening direction, by the vacuum of the vacuum chamber.

Based on this configuration, a poppet valve exposing a relatively large flow cross-section may be used with no need for it to work, during opening in the direction of the pressure chamber, against the pressure prevailing therein because a pressure equilibrium is provided to the effect that substantially the same pressure force acts upon the valve plunger not only from the side of the valve disc but also from the opposite side, such that the opening movement of the valve disc caused by the electromagnet is left substantially uninfluenced by the pressure difference between the pressure chamber and the vacuum chamber.

A structually advantageous form of embodiment is characterised in that, in the area of an opening provided in the piston wall, a valve body containing the connecting port is fixed to the piston wall. Thanks to this configuration, the valve can be mounted on the piston wall as a pre-assembled unit.

One practical form of embodiment is characterised in that the valve body is provided in the aperture of the piston wall and that the axis of the connecting port and the direction of movement of the valve disc extend perpendicularly to the plane of the piston wall. In particular, the actuating plunger may be provided with a central compensating port extending from the pressure chamber to a compensating chamber disposed behind the compensating piston and sealed against the vacuum chamber by a solid wall, and that the surface of the compensating piston facing away from the compensating chamber is in communication with the vacuum of the vacuum chamber.

Moreover, it is feasible for a compact arrangement of this embodiment to provide the electromagnet between the valve disc and the compensating piston on the valve body, and to locate a magnetic armature on the actuating plunger.

To avoid provision of a slidably displaceable compensating plunger within a piston, it is provided according to a most preferred embodiment, that the compensating piston, through an annular membrane absorbing longitudinal movements of the compensating piston, is circumferentially sealed against the solid wall. In particular, in view of the relatively low paths of displacemebnt, the compensating piston, feasibly, may be provided with a membrane rigidly connected to the valve body and the actuating plunger, respectively, and permitting longitudinal movements.

Adequate flow cross-sections behind the connecting port are provided in that between the valve disc and the vacuum chamber, connecting channels are circumferentially provided between the connecting port and the vaccum chamber.

Thanks to this configuration, the valve disc and the actuating plunger are free from static pressure forces. A compact arrangement is achieved because all actuating elements, the electromagnetic coil included, are located in the flow centre. Moreover, it is advantageous that the direction of movement of the valve disc and of the actuating plunger are perpendicular to the piston wall.

Another embodiment is characterised in that the short-circuiting valve is secured to the piston wall on the side of the pressure chamber with an actuating plunger extending in parallel to the piston wall, with a curved channel extending from the connecting port to a lateral passage bore of the valve body in registry with the aperture provided in the piston wall.

In particular, it is provided that the actuating plunger extends beyond the curved passageway down to the compensating piston at the surface of which facing away from the valve disc is a compensating chamber which, through a port, is in communication with the pressure chamber, whereas the surface of the compensating piston facing the valve disc is in communication with the curved channel.

Moreover, in respect of a compact arrangement favourable in terms of flow, it is advan- tageous for an actuating plunger to extend beyond the compensating piston into the electromagnet and to carry an armature therein.

Also, in that form of embodiment, feasibly, a compensating piston displaceable in a cylinder is not used, rather the compensating piston is in the form of a flexible membrane secured, radially inwardly, to the actuating plunger and, radially outwardly, to the valve body.

In all cases, it is feasible for a restoring spring to be provided between the surface of the compensating piston to which vacuum is applied, and a part rigidly secured to the valve body.

In the second embodiment described above, the valve throughput according to the first described embodiment may be increased by a power of ten. Preferably, a conical valve seat of a centring angle of 1500 is used thereby causing the flow forces to assist in the valve opening movement.

Also, in that case, the pressure on the valve disc and on the actuating plunger is balanced and the weight minimised.

The axis of the actuating plunger extends in parallel to the piston wall and, preferably, the valve body is disposed approximately one third in a trough of the piston wall.

Moreover, in this form of embodiment it is important for the electromagnet to be located within the pressure chamber, involving the advantage of an improved cooling as the air density within the pressure chamber is higher.

The flow within the valve body is deflected by 90" without exerting a reaction force in the closing direction on the movable valve disc.

Feasibly, the flow cross-section increases from about 200 to about 300 mm2 thereby causing a diffusor effect. The opening stroke of the valve may amount to, for example, 4 mm.

Before the short-circuiting valve, in the event of a braking skid control, is opened temporarily or in puise--type manner, the atmospheric pressure and the vacuum are to be separated, by means of quick-closing valves, from the pressure chamber and the vacuum chamber of the vacuum brake force booster, respectively.

It is, therefore, another object of the invention to provide a quick-acting valve closing particularly fast with the aid of the existing pressure forces.

Aence, the present invention, also, is concerned with a brake system for automotive vehicles, comprising a brake skid control and a vacuum brake force booster disposed between the brake pedal and a hydraulic master cylinder, wherein a fast-switching pneumatic short-circuiting valve to which pressure is applied by the control circuit of the brake skid control is provided in the piston wall of the brake force booster, which short-circuiting valve, during applying the brake pedal, upon occurrence of a braking skid between pressure and vacuum chambers, causes a pressure equilibrium reducing the braking skid, with quick-acting valves being additionally provided in the feed-in lines of the atmospheric pressure and/or the vacuum to the pressure chamber and vacuum chamber, respectively.

In order to permit utilisation of the pressure difference prevailing on the fast-acting valve for quickly closing the fast-acting valve it is provided, irrespective of the afore-described features, that a cup-shaped piston facing with the open side thereof the higher pressure, is axially displaceable in a cylinder, that the cupshaped piston, in the area of a bottom wall thereof, comprises circumferential passage ports terminating in an annular space provided between the cylinder and the cup-shaped piston, that an outlet opening is provided at a distance ahead of the bottom wall of the cupshaped piston in the front wall of the cylinder and is surrounded by an annular sealing surface co-operating with an annular seal on the bottom wall of the cup-shaped piston, and that a closing electromagnet is provided in the cylinder to act upon the cup-shaped piston.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of a brake system for automotive vehicles, comprising an hydraulic master cylinder and a vacuum brake force booster; Figure 2 shows, on enlarged scale, a sectional view of the piston wall of a first embodiment of a short-circuiting valve mounted on the piston wall; Figure 3 shows a sectional view of another embodiment of valve; Figure 4 shows a partial axial sectional view of a quick-closing valve useful for the brake system of automotive vehicles according to Figure 1, and Figure 5 shows a hydraulic switching system of the quick-closing valve according to Figure 4, wherein the flow through the quickclosing valve is always from the side of the cup-shaped piston.

According to Figure 1, pressure is applied by a brake pedal 12 of an automotive vehicle through a standard vacuum valve 45 to a vacuum brake force booster 14' provided with a mechanical travel-simulator 46 and including a movable piston wall 11' separating a pressure chamber 16' from the vacuum chamber 17'.

The vacuum chamber 17', through a conduit 47, is in communication with an intermediate vacuum tank 48 kept at a predetermined vacuum through a vacuum source 49. The vacuum source 49, feasibly, is the intake-tube of the internal combustion engine of the automotive vehicle.

The pressure chamber 16', through the standard vacuum valve 45, in known manner, can be placed in communication with atmospheric pressure thereby displacing the piston wall 11' by the extent to which the brake pedal 12 is pressed down.

The pressure chamber 16' and the vacuum chamber 17', through respective quick-closing valves 50, are in communication with the pressure chamber 16 and the vacuum chamber 17 of a vacuum brake force booster 14 of larger diameter. The quick-closing valves 50 are actuated by closing electromagnets 44 which are connected to a control circuit 15 generating closing signals for the quick-closing valves 50 in the event that a braking slip control should be required.

Moreover, the electromagnet 21 of a shortcircuiting valve 18 is applied to the control circuit 15 provided for the braking skid control, which short-circuiting valve 18 can establish a communication between the pressure chamber 16 and the vacuum chamber 17 of the vacuum brake force booster 14. The short-circuiting valve 18 is-as individually disclosed in Figures 2 and 3-in fact, located within the piston wall 11.

Figure 1 shows the position of the valves 50 and 18, respectively, in the event that no braking skid control takes place. Once the brake pedal 12 is pressed down, the pressure within the pressure chamber 16 and 16', respectively, increases and the piston walls 11 and 11' in Figure 1 move to the right, with the central plunger 51 of the piston wall 11 applying pressure to the pistons 52 of a tandem master cylinder 53 to develop the required hydraulic pressure in the hydraulic conduits 54,55 leading to the brake conduits.

If, during a braking operation, a braking skid signal is provided to the control circuit 15 by a braking skid sensors 55', which is generated upon commencement of a locking of one of the wheels, the control circuit 15, through the closing electromagnets 44, causes a quick closure of the fast-acting valves 50 and an opening of the short-circuiting valve 18. Preferably, the opening of the short-circuiting valve 18 is in a pulse-type manner, with a more or less substantial pressure compensation occurring between the pressure chamber 16 and the vacuum chamber 17 until the braking skid on the wheel concerned is removed, whereafter the short-circuiting valve 18 will re-close and the quick-acting valves 50 re-open.

According to Figure 2, the short-circuiting valve 18 comprises a rotationally-symmetrical valve body 24 having a centre axis 83 standing upright on (perpendicular to) the piston wall 11. The valve body 24 is tightly secured in a circular opening 25 of the piston wall 11.

In the area projecting into the pressure chamber 16, the valve body 24 comprises a largearea, circular connecting port 19 upon which is sealingly mounted from the pressure chamber 16 side a valve disc 20 radially extending across the connecting port 19, with the rim of the connecting port 19 forming a valve seat 84.

The valve disc 20, with a cylindrical projection 13 projecting away from the pressure chamber 16, is slidably seated on a guide pin 56 of the valve body 24. Provided circumferentially of the projection 13 is a broad flow passageway disposed circumferentially of the connecting channels 31 terminating beyond the piston wall 11 in the vacuum chamber 17.

Concentrically secured on the side of the valve body 24 facing the vacuum chamber 17 is an electromagnet 21 in the interior of which is located a sleeve-type magnetic armature 29 which is firmly disposed on the actuating plunger 22 of the valve disc 20.

The actuating plunger 22 is rigidly and centrally connected to the valve disc 20 and extends through a central port 57 of the valve body 24 and a central port 58 of the electromagnet 21 and into the latter where the sleeve-type magnetic armature 29 is mounted thereon.

The actuating plunger 22 on the side facing away from the pressure chamber 16, markedly, projects beyond the electromagnet 21 and the armature 29 to carry a compensating piston 23. A restoring spring 34 extends between the electromagnet 21 and the compensating piston 23.

The actuating plunger 22 comprises a continuous central compensating port 26 interconnecting the pressure chamber 16 and a compensating chamber 27 provided behind the compensating piston 23, which compensating chamber 27 is sealed by a solid wall 28. The compensating piston 23 is tightly secured by a radially outwardly guided annular membrane 30 in relation to the circumference of the solid wall 28. The annular membrane 30 permits the required axial movement of the compen sating piston 23.

Circumferentially of the solid wall 28, a cap 59 including passage ports 60 toward the vacuum chamber 17, extends to the circumference of the electromagnet 21 and is suitably attached thereto. The solid wall 28, hence, forms an integral component with the valve body 24.

The diameter of the compensating piston 23, the effective pressure surface of the annular membrane 30 included, corresponds to the diameter of the valve port 19.

Figure 2 shows the rest position of the short-circuiting valve 18. Once the electromagnet 21 is energised, the armature 29 slightly projecting in the rest position toward the vacuum chamber 17 from the electromagnet 21, is drawn into the electromagnet 21 thereby moving the actuating plunger 22 in Figure 2 to the left and removing the valve disc 20 from the valve seat circumferentially of the valve port 19. Now, air can flow from the pressure chamber 16 around the valve disc 20 into the vacuum chamber 17 as long as the short-circuiting valve 18 is open.

Once the electromagnet 21 is cut off, the restoring spring 34 restores the actuating plunger 22 and the valve disc 20 into the closing position as shown in Figure 2.

In the closing position of Figure 2, the pressure in the pressure chamber 16 acts upon the valve disc 20 in the direction of arrow F.

However, thanks to the central bore 26 in the actuating plunger 22, the said pressure also acts in the compensating chamber 27 such that equal pressure acts upon the compensating piston 23 from the direction opposite the direction of arrow F, thereby neutralising the two pressure forces equal in opposite directions, on the actuating plunger 22, enabling the latter to operate independently of the pressure difference between the pressure chamber 16 and the vacuum chamber 17.

However, this will require that the surface of the compensating piston 23 facing the electromagnet 21 as well as the back of the valve disc 20, be exposed to the vacuum in the vacuum chamber 17, this is ensured by the ports 60 circumferentially located in the cap 59 and terminating in the vacuum chamber 17.

In the embodiment according to Figure 3, identical numerals designate corresponding parts to those shown in Figure 2.

According to Figure 3, the actuating plunger 22 is disposed in parallel with the piston wall 11 which is provided with a depression 61 on the side facing the pressure chamber 16. The valve body 24 comprises a lateral passage port 32 adjacent the aperture 25 provided in the piston wall 11 and in registry therewith.

From the passage port 32, a curved channel 33, slightly tapering, at an angle of 90 , leads to the valve bore 19 including valve seat 84, which bore is externally closed by the valve disc 20. The actuating plunger 22 extends through a central bore 57 of the valve body 24 and through a port 62 provided in a partition 63 located behind the channel 33 down to the compensating piston 23 which, in this embodiment, is in the form of a completely flexible membrane fixed radially inwardly to the actuating plunger 22 and radially outwardly to the circumference of a correspondingly large aperture 64 provided in the valve body 24.

Behind the membrane forming the compensating piston 23, a rounded mounting disc 65 is fixed to the actuating plunger 22 to support the membrane during gagging in a direction opposite the direction shown in Figure 3.

The restoring spring 34 extends between the partition 63 and the membrane forming the compensating piston 23, with a spring abutment 66 covering the membrane being additionally provided between the membrane and the restoring spring 34.

The actuating plunger extends beyond the compensating piston 23 through a central port 58 of the electromagnet 21 into the latter where the armature 29 is secured therein. Located between the compensating piston 23 and the electromagnet 21 is a compensating chamber 27 which, through a port 26', is in communication with the pressure chamber 16.

The chamber containing the restoring spring 34, through the port 62, is in communication with the interior of the curved channel 33 such that the pressure of the vacuum chamber 17 prevails therein.

The operation of the embodiment according to Figure 3 corresponds to the embodiment according to Figure 2, with a pressure compensation equally taking place on the actuating plunger 22, provided that the face of the compensating piston 23 corresponds to the face of the valve bore 19.

Figure 4 shows a preferred quick-closing valve 50 corresponding to one suitably used in the brake system for automotive vehicles as shown in Figure 1.

According to Figure 4, a cup-shaped stepped piston 35 is axially displaceable in a stepped cylinder 36. The bottom wall 37 of the cup-shaped piston 35 is disposed opposite an outlet opening 40 at a distance defining the opening gap of the quick-closing valve 50, with the outlet opening 40 being centrally provided in the front wall 41 of the cylinder 36 which, in Figure 4, is the right-hand wall, and, on the side facing the cup-shaped piston 35, is surrounded by an annular sealing face 42. An annular seal 43 for co-operation with the annular sealing face 42 is provided axially opposite the annular sealing face 42 at the bottom wall 37 of the cup-shaped piston 35 in concentric relationship to the outlet opening 40.

Provided in the circumferential area of the cup-shaped piston 35 next to the bottom wall 37 are passage ports 38 obliquely extending circumferentially to the direction of flow (arrow f) and terminating in an annular chamber 39 formed by the inside diameter of the cylinder 36 at this point being markedly larger than the outer diameter of the cup-shaped piston 35.

In the area behind the passage ports 38 the remainder of the cup-shaped cylinder 35 is disposed in a manner closely and axially displaceable within the cylinder 36.

Directly behind the passage ports 38, the cylinder 36 expands in a step-type manner radially outwardly, and provided behind the step 67 is a guiding ring 68 bridging the annular chamber 39 radially inwardly and externally, and being in sliding abutment with the cup-shaped piston 35.

At a distance 69 from the guide ring 68, the cup-shaped piston 35 projects radially outwardly through an annular step 70 to reach, at that point, approximately the inside diameter of the front part of the cylinder 36.

The rear end of the cup-shaped piston 35 comprises a radially outwardly extending flange 71 which is in axial abutment with a ring-shaped projection 72 of the stepped cylinder 36.

Disposed circumferentially of the cup-shaped piston 35, in expansion 73 of the cylinder 36, is a closing electromagnet 44 which, upon energisation, acts, in the closing direction, on the cup piston 35 which may be of a magnetic material. Moreover, opposite the direction of flow f, an opening electromagnet 74 may be additionally provided behind the cup piston 35.

Disposed circumferentially of the flange 71, in abutting relationship, is a spring abutment ring 75, preferably of magnetic material, with a restoring spring 76 extending between the latter and the guide ring 68.

Optionally, a weak auxiliary spring 78 serving to release the closing movement, may be additionally provided between an inner annular stop 77 disposed in the rear area of the cylinder 36, and the rear surface of the flange 71.

Figure 4 shows the open position of the quick-closing valve 50 which is defined by the restoring spring 76, wherein the flow fluid flows from the pressure side 79, in the direction of arrow f, through the passageways 38 to the outlet opening 40.

The flow cross-section is reduced from the valve inlet to increase the flow speed and to decrease the static pressure. In the accumulating space 80, ahead of the passage ports 38, thereby a static pressure will develop which acts in the closing direction of the quick-closing valve 50.

Once the closing electromagnet 44 is energised, either the cup-shaped piston-when magnetically configured-or only the spring abutment 75 all of a sudden is displaced in the direction of the arrow f, with the cupshaped piston either by itself or through the force of flow or, in addition, through the magnetic force, all of a sudden being moved, in the closing direction of Figure 4, to the right wherein the annular seal 43 is sealingly mounted onto the sealing face 42.

After de-energisation of the closing electromagnet 44, the restoring spring 76 restores the cup-shaped piston 35 through the spring abutment ring 75 back to the initial position.

This restoring movement may be assisted by energising the opening electromagnet 74.

It should be noted that the magnetic circuit will not have to accelerate the mass of the whole of the valve body but will rather only have to move the spring abutment ring 75 against the relatively weak restoring spring 76. The opening process of the valve will solely have to be performed by the spring force for which reason corresponding anti-adhesive foils are provided in the magnetic circuit. Should this be inadequate, the opening electric magnet 74 assisting in opening would be additionally provided.

It is a prerequisite for the smooth operation of the quick-closing valve 50 according to Figure 4 that the flow therethrough is always -in the direction of the arrow f. A switching action in a reversed flow direction is not readily insured. However, the hydraulic circuit 81 according to Figure 5 is capable of safeguarding that the flow through the valve 50 is always in the direction of arrow f. For that purpose, the quick-closing valve 50 is provided on a hydraulic ring system containing four check valves 82 switched in accordance with the illustration in Figure 5. Depending on the direction of flow (arrow in solid lines and arrow in dashed lines, respectively), the check valves 82 respectively open or close automatically such that the flow through the quick-closing valve 50 is always in the same direction f.

Claims

1. A brake system, for automotive vehicles, comprising a braking skid control and a vacuum brake booster disposed between a brake pedal and an hydraulic master cylinder, wherein a quick-acting pneumatic short-circuiting valve to which pressure is applied by a control circuit of the braking skid control is provided in a piston wall of the booster, which short-circuiting valve, during a braking operation and upon occurrence of a braking skid, causes a pressure compensation between a pressure chamber and a vacuum chamber, reducing the braking skid, characterised in that the short-circuiting valve (18) comprises a valve disc covering a connecting bore (19) between the pressure chamber (16) and the vacuum chamber (17) from the pressure chamber (16) side and being removable from the connecting port (19) by means of an electromagnet (21) in the direction of the pressure chamber (16), that is the opening di rection, and that the actuating plunger (22) of the valve disc (20) in a direction opposite the opening direction extends to a compensating piston (23) provided thereon and acted upon, from the side opposite the opening direction, by the pressure of the pressure chamber (16) and, from the side facing the opening direction, by the vacuum of the vacuum chamber (17).

2. A system according to claim 1, characterised in that a valve body (24) containing the connecting port (19) is, in the area of an aperture (25) provided in the piston wall (11), fixed to the piston wall (11).

3. A system according to claim 2, characterised in that the valve body (24) is located in the aperture (25) of the piston wall (11), and that the axis of the connecting port (19) and the direction of movement of the valve disc (20) extend perpendicularly to the plane of the piston wall (11).

4. A system according to claim 3, characterised in that the actuating plunger (22) is provided with a central compensating port (26) extending from the pressure chamber to a compensating chamber (27) located behind the compensating piston (23) and sealed against the vacuum chamber (17) by a solid wall (28), and that the surface of the compensating piston (23) facing away from the compensating chamber (27) is in communication with the vacuum of the vacuum chamber (17).

5. A system according to claim 3 or claim 4, characterised in that disposed between the valve disc (20) and the compensating piston (23), on the valve body (24), is the electromagnet (21) and, on the actuating plunger (22), is a magnetic armature (29).

6. A system according to claim 4 or claim 5, characterised in that the compensating piston (23), through an annular membrane (30) absorbing longitudinal movements of the compensating piston (23) is circumferentially sealed against the solid wall (28).

7. A system according to any one of claims 3 to 6, characterised in that circumferentially provided between the valve disc (20) and the vacuum chamber (17) are connecting channels (31) disposed between the connecting port (19) and the vacuum chamber (17).

8. A system according to claim 1 or claim 2, characterised in that the short-circuiting valve (18), on the sjde of the pressure chamber (16), with an actuating plunger (22) extending in parallel tq the piston wall (11), is fixed to the piston wall (11), with a channel (33), preferably curved, extending from the connecting port (19) to a lateral passage port (32) of the valve body (24) in registry with the aperture (25) in the piston wall (11).

9. A system according to claim 8, characterised in that the actuating plunger (22) extends beyond the curved channel (33) to the compensating piston (23) at the surface of which facing away from the valve disc (20) is a compensating chamber (27) which, through a port (26') is in communication with the pressure chamber (16), whereas the surface of the compensating piston (23) facing the valve disc (20) is in communication with the curved channel (33).

10. A system according to claim 9, characterised in that the actuating plunger (22) extends beyond the compensating piston (23) into the electromagnet (21) to carry an armature (29) therein.

11. A system according to any one of claims 8 to 10, characterised in that the compensating piston (23) is in the form of a flexible membrane which, radially inwardly, is fixed to the actuating plunger (22) and, radially outwardly, is attached to the valve body (24).

12. A system according to any one of the preceding claims, characterised in that a restoring spring (34) is disposed between the surface of the compensating piston (23) applied by the vacuum, and a part rigidly connected to the valve body.

13. A system according to any one of the preceding claims, characterised in that the surface area of the compensating piston substantially corresponds to the surface area of the connecting port (19).

14. A system, in particular according to any one of the preceding claims, comprising quickclosing valves in feed-in lines of the atmospheric pressure and/or of the vacuum to the pressure chamber and the vacuum chamber, respectively, characterised in that a cupshaped piston (35) facing, with the open side thereof, the higher pressure is axially displaceable in a cylinder (36), that the cup-shaped piston (35), in the area of a bottom (37) thereof, comprises circumferential passage ports (38) terminating in an annular chamber (39) provided between the cylinder (36) and the cup-shaped piston (35) > that an outlet opening (40) is provided at a distance ahead of the bottom wall (37) of the cup-shaped piston (35) in the front wall (41) of the cylinder (36) and surrounded by an annular sealing surface (42) in co-operation with an annular seal (43) on the bottom wail (37) of the cupshaped piston (35), and that a closing electromagnet (44) acting upon the cup-shaped piston (35) is provided in the cylinder (36).

15. A brake system, for automotive vehicles, substantially as herein described with reference to Figures 1 and 2 or Figures 1 and 3, with or without reference to Figure 4 or Figure 5, of the accompanying drawings.