Hydraulic vehicle brake system with an anti-skid system

A hydraulic motor vehicle brake system having anti-skid system and further developed to improve vehicle tracking, especially during cornering by means of automatic braking. With automatic braking an auxiliary pressure source delivers a pressure medium to an anti-skid system, by means of which braking pressures can be individually adjusted in the wheel brakes. For this purpose a cylinder with separating piston sections is located between the auxiliary pressure source and the anti-skid system, wherein the separating piston is installed into 2/2-way valves including lip sealing rings. The embodiment in accordance with the invention include cylinders, with valve openings in cooperation with the lip sealing rings, which results in economical 2/2-way valves.

FIELD OF THE INVENTION 
The invention is based on a hydraulic brake system as defined hereinafter. 
BACKGROUND OF THE INVENTION 
A hydraulic vehicle brake system with an anti-skid system and a control 
device is known from U.S. Pat. No. 4,809,183. Furthermore, a hydraulic 
pressure source and at least one evaluating circuit are associated with 
the anti-skid system for detecting an oversteering tendency, an 
understeering tendency and the start of the danger of skidding, and for 
generating control signals for the anti-skid system, so that this system 
automatically brakes the vehicle with the use of the hydraulic pressure 
source, and from case to case the wheels of one wheel axle or of all wheel 
axles are braked. The automatic braking stabilizes the vehicle, thus 
counteracting an oversteering tendency, an understeering tendency, or a 
danger of skidding, or skidding that is already in progress. The pressure 
source is not shown. To obtain the control signals, king pin inclinations 
are measured, for example, and compared with preselected threshold values. 
The rotating velocity of the vehicle around the vertical axis can also be 
observed by means of a gyroscope, for example. Alternatively, transverse 
accelerations can be measured by means of acceleration sensors associated 
with the axles and evaluated. 
German Patent Disclosure DE 41 09 025 A1 discloses a hydraulic vehicle 
brake system with advantageous, closed brake circuits and an anti-skid 
system that operates in accordance with the so-called recalculating pump 
principle, and in which a combination of a controllable 2/2-way valve and 
a cylinder with a separating piston and a restoring spring for the 
separating piston is installed for each brake circuit, between this 
anti-skid system and a main brake cylinder, to control the cornering of a 
vehicle; a common auxiliary pump and a 3/2-way valve disposed downstream 
of it are also provided for both brake circuits for controlled charging of 
the separating piston and to control the 2/2-way valves, which are 
hydraulically controllable by means of control inputs for closing the 
brake lines between the cylinders and the main brake cylinder. Pressures 
generated in the cylinders after the main brake lines have been blocked 
are modulated by means of the anti-skid system and supplied individually 
to the wheel brakes of the four wheels. The anti-skid system can also 
individually reduce the pressures supplied to the wheel brakes. A control 
device that controls both the 3/2-way valve and the anti-skid system to 
improve the cornering of the vehicle equipped in this manner can also 
activate a driving motor for the auxiliary pump. The disadvantage is that, 
because of the hydraulic connection of the cylinders with modulation valve 
arrangements of the anti-skid system, the auxiliary pump must deliver a 
pressure at the level of the highest brake pressure to be expected during 
cornering. Correspondingly, the auxiliary pump must be embodied to be 
powerful; a required driving motor is heavy and stresses an electrical 
system of the vehicle in a disadvantageous manner. The heavy driving motor 
and the heavy auxiliary pump are expensive. A further disadvantage is 
that, because of the high pressure generated, disturbing and possibly very 
aggravating noises occur during pump operation. A reservoir that can be 
charged by the auxiliary pump is also expensive, as is the electrically 
controllable 3/2-way valve that controls the charging of the separating 
piston and the hydraulic control inputs of the 2/2-way valves. Moreover, 
because of the embodiment of the cylinder and its separating piston, the 
respective 2/2-way valve must be combined with a check valve function, so 
that during automatic brake operation the transition into normal brake 
operation can immediately take place solely through operation of the brake 
pedal. Therefore, in the disclosed exemplary embodiment the symbol of a 
check valve is allocated to the second position of the 2/2-way valves. 
It is known from U.S. Pat. No. 4,412,701 to further embody a hydraulic 
vehicle brake system with a main brake cylinder and wheel brake cylinders 
for automatic distance control from a preceding vehicle, and/or to limit 
wheel slip. For this purpose a cylinder with a separating piston embodied 
as a stepped piston and displaceably disposed therein is installed for 
each brake circuit, between the main brake cylinder and the wheel brake 
cylinders. An auxiliary pressure source is associated with the cylinders. 
The separating piston has a lip sealing ring in the region of its large 
diameter, and there defines a primary chamber that can be charged by 
pressure from the auxiliary pressure source. In the region of the opposite 
end of the separating piston, which has a smaller diameter, the separating 
piston has a further lip sealing ring that defines a secondary chamber 
within the cylinder. A valve opening in the wall of the cylinder is 
associated with this lip sealing ring. When the separating piston is 
displaced, this lip sealing ring travels across the valve opening and, in 
this way, forms a 2/2-way valve with the opening. The valve opening is 
connected to the main brake cylinder. The secondary chamber: communicates 
with wheel brake cylinders. In this respect, braking is possible on the 
one hand by means of pressure from the main brake cylinder by operating a 
brake pedal and, on the other hand, automatically by means of introducing 
auxiliary pressure from the auxiliary pressure source into the primary 
chamber, wherein the auxiliary pressure displaces the separating piston, 
closes the 2/2-way valve and generates the required brake pressure in the 
secondary chamber. A third lip sealing ring whose sealing lip likewise 
points toward the secondary chamber is disposed between the two lip 
sealing rings. Between the third lip sealing ring and the lip sealing ring 
on the side of the primary chamber, the cylinder has a pressure 
equalization opening open to the outside. It is disadvantageous that 
leakage caused by wear or damage to the lip sealing ring associated with 
the valve hole cannot be detected during normal braking operations; 
rather, it is first detected when automatic braking is intended to be 
initiated, and either no braking effect or a braking effect of inadequate 
duration results. When this leakage is present, the middle lip sealing 
ring should assure normal braking with the operation of the main brake 
cylinder. Whether the middle lip sealing ring itself is leaky cannot be 
detected during automatic braking, and can only be detected by chance with 
normal braking by means of the main brake cylinder when the main brake 
cylinder and the lip sealing ring associated with the valve hole are 
leaky. If both seals are leaking, the additional disadvantage arises that 
the pressure medium flows around both lip sealing rings, and the pressure 
medium escapes into the environment through the pressure equalization 
opening. By means of this, the vehicle brake system is emptied, so that a 
braking effect can no longer be generated. 
German Patent Application P 42 32 311.8 (DE 42 32 311 A1) proposes to 
provide a hydraulic vehicle brake system designed for automatic braking 
for the purpose of improving the driving performance of a vehicle (driving 
dynamics control) and/or traction control with a cylinder with a 
separating piston displaceably disposed therein, and an auxiliary pump for 
automatically charging the separating piston whose pressure is 
significantly lower than the full brake pressure generated in the main 
brake cylinder, between a main brake cylinder connection and an anti-skid 
system, that operates in accordance with the so-called recirculating pump 
principle. To assure operational reliability and monitoring capability 
with regard to operational reliability, the separating piston has two 
piston sealing rings. Moreover, the piston, connected to a conduit leading 
out between the piston rings and communicating with the main brake 
cylinder, includes a 2/2-way valve embodied as a seat valve with a ball 
embodied as a sealing element that is closed when the piston is displaced 
by means of auxiliary pressure. A 2/2-way valve in seat design, built into 
the separating piston, is expensive. A further exemplary embodiment 
proposed by this German patent application P 42 32 311.8 (DE 42 32 311 
A1), with a cylinder and a 2/2-way valve, additionally includes a floating 
piston that likewise has two piston sealing rings for reasons of 
operational reliability. This exemplary embodiment is even more expensive 
than the first exemplary embodiment mentioned. In both exemplary 
embodiments it is provided that the cylinders can be connected via 
electrically operable 2/2-way valves to intakes of the recirculating pumps 
for the purpose of automatic braking, so that the recirculating pumps can 
exert high pressure for braking on the pressure medium originating from 
the cylinders. 
OBJECT AND SUMMARY OF THE INVENTION 
The hydraulic vehicle brake system of the invention has the advantage that 
lower technological expenditures are required for the cylinder and the 
separating piston and its 2/2-way valve than in the example of German 
Patent Application P 42 32 311.8 (DE 42 32 311 A1), and that, in contrast 
to the example of a cylinder and separating piston with three seal rings 
in accordance with U.S. Pat. No. 4,412,701, in case of leakage of the lip 
sealing rings associated with the secondary chamber, those quantities of 
pressure medium that trickle through because of the operation of the main 
brake cylinder are supplied to the reservoir of the main brake cylinder. 
As a consequence, the hydraulic vehicle brake system embodied in a way in 
accordance with the invention is itself protected against emptying when 
both of the lip sealing rings associated with the secondary chamber are 
leaking, and also when the lip sealing ring associated with the primary 
chamber is leaking. Thus, the possibility arises of achieving at least a 
short-time braking effect through arbitrarily frequent application of the 
brake pedal. The cylinder with the separating piston designed in 
accordance with the invention can, of course, also be inserted into a 
brake system in accordance with the document mentioned in the introduction 
to the description, DE 41 09 025 A1. 
An advantageous further development and improvement of the vehicle brake 
system disclosed is possible by means of the provision outlined herein. 
The features set forth result in the advantage that the second 2/2-way 
valve is closed before the lip sealing ring of the first 2/2-way valve 
passes across the associated valve opening, by means of which an undesired 
discharge of pressure medium from the main brake cylinder into the 
reservoir is prevented. 
The invention will be better understood and further objects and advantages 
thereof will become more apparent from the ensuing detailed description of 
a preferred embodiment taken in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The exemplary embodiment in FIG. 1 of the hydraulic vehicle brake system of 
the invention has a dual-circuit main brake cylinder 3 with a reservoir 4 
that supplies it and a pneumatic power brake 5, for example, as well as a 
brake pedal 6, two brake circuits I and II, front-wheel brakes 7 and 8 
associated with brake circuit I, rear-wheel brakes 9 and 10 associated 
with wheel circuit II, an anti-skid system 11 disposed between the main 
brake cylinder 3 and the wheel brakes 7 through 10, and an additional 
device 12 for automatic braking. 
The main brake cylinder 3 is embodied in a way that is known per se, and is 
operable with the aid of the brake pedal 6. The effect of brake pedal 6 on 
the main brake cylinder 3 can be increased by the power brake 5. The 
reservoir 4 supplies the main brake cylinder 3 with a pressure medium 
that, for normal braking operation, can be pushed from the main brake 
cylinder 3 in the direction of the wheel brakes 7 through 10, through the 
additional device 12 and the anti-skid system 11. 
The anti-skid system 11 is an anti-skid system of the so-called 
recirculating pump type, and has a recirculating pump 13 for the brake 
circuit I and a recirculating pump 14 for the brake circuit II. Further, 
the anti-skid system 11 has a brake pressure modulation valve arrangement 
15, 16, 17 or 18, comprised of a brake pressure buildup valve 19 and a 
respective braking pressure reduction valve 20, for each wheel brake 7 
through 10, as well as a reservoir 21 or 22 for each brake circuit I and 
II. Moreover, a first damping chamber 23 for the brake circuit I, for 
example, and a second damping chamber 24 for the brake circuit II and a 
damping throttle 25 or damping throttle 26 are provided. A motor 27 is 
associated as the drive with there circulating pumps 13 and 14. Starting 
at the wheel brakes 7 through 10, their respectively associated brake 
pressure buildup valves 19 can be bypassed in the direction toward the 
main brake cylinder 3 by means of check valves 28, which can be opened in 
the direction toward the main brake cylinder 3 in case of a drop in 
pressure above the respective brake pressure buildup valve 19, for example 
when this valve is switched into its blocked position, or when an 
installed throttle is effective when the valve is in its open position. 
Anti-skid operation is possible by means of the said individual elements 
for the anti-skid system 11. For this purpose the brake pressure buildup 
valves 19 are normally in the open position and located between the 
respective wheel brakes 7 through 10 and the main brake cylinder 3, so 
that pressures generated in the main brake cylinder 3 can normally travel 
into the wheel brakes 7 through 10 by means of operation of the brake 
pedal 6. Respective braking pressure decrease valves 20 of the brake 
pressure modulation valve arrangements 15 and 16 or 17 and 18, likewise 
connected to the wheel brakes 7 through 10, are closed in the normal 
position and permit a throttling passage in the activated position, and 
communicate on the inlet side with the recirculating pump 13 of the brake 
circuit I or on the inlet side with the recirculating pump 14 of the brake 
circuit II. The damping chambers 23 or 24 are connected on the outlet side 
of the recirculating pump 13 or 14. The throttles 25 or 26 follow the 
damping chambers 23 or 24 in the direction toward the main brake cylinder 
3 and the respective brake pressure buildup valves 9. 
A further control device 30, shown in outline, is also part of the 
anti-skid system 11, as are the wheel revolution sensors 31 through 34 
associated with the wheels, not shown, of the wheel brakes 7 through 10, 
The control device 30 is connected to these wheel revolution sensors 31 
through 34. The individual valves 19, 20 of the brake pressure modulation 
valve arrangements 15 through 18 are connected on one side to the control 
device 30 itself, and the motor 27 is connected to the control device on 
the other side. Furthermore, a brake pedal switch 35 and/or a pressure 
sensor 36, for example, that are connected to the control device 30 can be 
disposed in the region of the brake pedal 6. 
Through the operation of the brake pedal 6, with the aid of the power brake 
5, for example, pressure is generated in the main brake cylinder 3 that is 
propagated to the wheel brakes 7, 8, 9 and 10 through the open brake 
pressure buildup valves 19 of the two brake circuits I and II. 
If the brake pedal 6 is increasingly strongly applied on a uniformly 
non-skid road surface, for example, the sequences of signals emitted by 
the wheel revolution sensors 31 through 34 are changed in a way that is 
ultimately interpreted by the control device 30 as a danger of wheel 
locking. Assuming that the danger of wheel locking threatens at all wheels 
at the same time and with the same intensity, the control device 30 
activates the motor 27 and therefore the recirculating pumps 13 and 14, 
closes all of the brake pressure buildup valves 19 of the brake pressure 
modulation valve devices 15 through 18, and opens all associated braking 
pressure decrease valves 20. The consequence of this is that pressure that 
might still be increasing in the main brake cylinder 3 does not reach the 
wheel brakes 7 through 10; instead, quantities of pressure medium flow out 
of these wheel brakes to the storage chambers 21 or 22. As a consequence, 
these recirculating pumps 13 and 14 force pressure medium through the 
damping chambers 23 and 24 and the throttles 25 and 26, and through the 
two brake circuits I and II, back to the main brake cylinder 3. After the 
danger of wheel locking has ceased, assuming that the danger of wheel 
locking happens to disappear simultaneously for all of the wheels 7 
through 10, the brake pressure modulation valve arrangements 15 through 18 
are brought into their base positions, in that the control device 30 halts 
the supply of control currents necessary for the decrease in braking 
pressure. If, during a subsequent, preselected period of time no further 
danger of wheel locking occurs, for example, then the control device 30 
also halts the current supply to the motor 27. 
In a way known per se, for example, the control device 30 is also designed 
such that pressures of the front-wheel brakes 7 and 8 can be modulated 
individually and independently of one another, so that the best possible 
braking effect of the front wheels can be generated on roadways with 
partially varying frictional values. For instance, a low frictional value 
.can be present to the right at the edge of a road as a consequence of ice 
formation, and a high frictional value can be present to the left under 
the vehicle across a dry stretch of road. 
An individual control or regulating principle of this type can also be 
applied for the rear-wheel brakes 9 and 10, but a so-called "select-low" 
control, for example, is also used under certain conditions, i.e., if one 
of the rear wheels threatens to lock, the braking pressure in the wheel 
brake of the other rear wheel is also reduced. By means of this, the 
contribution made by the rear-wheel brakes to vehicle deceleration is 
reduced if need be; howevers this can be of great advantage because it 
favors lateral guiding forces during the execution of a curve. In this 
case a favorable effect on driving performance is meant in the sense that 
a lateral drifting of the rear wheels out of the curve can be suppressed. 
The said "select-low" control of the rear-wheel brakes 9 and 10 is 
therefore a useful measure for affecting the motion pattern of the 
vehicle, particularly the rotating behavior around the center of gravity 
or the vertical axis of the vehicle. An undesired angular deceleration of 
the vehicle around the vertical axis, that is, inherent dynamics of the 
vehicle, can thus be counteracted in the described manner. 
A cylinder 237 or 238, a reversing valve 39 or 40, a pressure-limiting 
valve 41 or 42 and, for safety reasons, a check valve 43 or 44 are part of 
the device 12 for automatic braking inside the brake circuits I and II, 
respectively. Moreover, 2/2-way valves 45 and 46 as well as check valves 
91 and 92 are associated on the inlet side with the recirculating pumps 13 
or 14. For indirect supply, an auxiliary pump 49 with an auxiliary motor 
50, an auxiliary pressure-limiting valve 51 and a throttle 52 are also 
part of the device 12. 
The auxiliary motor 50 is connected to the control device 30 and can be 
activated by the control device 30, so that the auxiliary pump 49 can be 
driven. The auxiliary pump 49 has an inlet 53, which in the example is 
connected to the reservoir 4 of the main brake cylinder 3 via a line 54. 
An outlet 55 is connected to the two cylinders 237 and 238 via an 
auxiliary pressure line 56. The auxiliary pressure-limiting valve 51 is 
connected on the inlet side to the outlet 55 of the auxiliary pump 49, and 
can be opened in the direction toward the inlet 53 of this auxiliary pump 
49 or the line 54, which communicates with the reservoir 4. The auxiliary 
pressure-limiting valve 51 is designed such that it can open by means of a 
pressure with a magnitude of 5 to 10 bar, for example. The throttle 52 is 
likewise connected to the outlet 55 of the auxiliary pump 49 and 
communicates on the other side with the line 54 and the inlet 53 of the 
auxiliary pump 49. When the auxiliary pump 49 is stopped, the throttle 52 
assures the elimination of differences in pressure between the outlet 55 
and the inlet 53. The listed elements form a controllable auxiliary 
pressure source 57 because of the cooperation of the control device 30 
with the auxiliary motor 50. 
Cylinders 237 and 238 are embodied identically. Therefore, it is only 
necessary to describe the cylinder 237 in conjunction with FIG. 2. The 
cylinder 237 has an end wall 239, and is sealed on the side opposite this 
wall by means of a screwed-on threaded lid 240. A sealing ring 241 is 
clamped between this lid 240 and the cylinder 237. A piston 242 is 
displaceably disposed inside the cylinder 237. The piston 242 has a 
beginning end 243 adjacent to the lid 240 and an ending end 244 located 
opposite thereto. A primary chamber 245 is located inside the cylinder 
237, adjacent to the lid 240 and the beginning end 243. A secondary 
chamber 246 is located between the ending end 244 of the piston 242 and 
the end wall 239. The piston 242 is intended to separate the secondary 
chamber 246 from the primary chamber 245, and is therefore designated as a 
separating piston. A piston restoring spring 247 is located in the 
secondary chamber 246. So that a fluid-proof separation of the secondary 
chamber 246 from the primary chamber 245 occurs, the separating piston 242 
is provided in the region of its beginning end 243 with a lip sealing ring 
249 inserted into a circumferential groove 248. In this case a lip 250 of 
the lip sealing ring 249 points toward the lid 240. To seal the secondary 
chamber 246, a further lip sealing ring 251 is inserted in the region of 
the ending end 244 of the separating piston 242 into a circumferential 
groove 252 cut into the separating piston 242. This lip sealing ring 251 
also has at least one lip 253, which points toward the end wall 239 and is 
therefore suited to contain a pressure to be generated in the secondary 
chamber 246. A valve opening 254 is associated with the lip sealing ring 
251. This is embodied in the way known for main brake cylinders. In this 
case the valve opening 254 starts from a connection 256 incorporated into 
the cylinder 237. The connection 256 is connected to the main brake 
cylinder 3 via the brake circuit II. Therefore the secondary chamber 246 
communicates with the main brake cylinder 3 in the base position of the 
separating piston 242. A further connection 257 for the secondary chamber 
246 passes through the end wall 239, for example, for a connection to the 
anti-skid system 11. It can be recognized, with reference to a known main 
brake cylinder, that with a displacement of the separating piston 242 in 
the direction toward the end wall 239, the lip 253 passes across the valve 
opening 254, thus hydraulically separating the secondary chamber 246 from 
the main brake cylinder 3. To this extent, the lip 253 and the valve 
opening form a controllable 2/2-way valve. Thus, when a pressure is 
generated inside the secondary chamber 246 by means of the displacement of 
the separating piston 242, this pressure cannot escape to the main brake 
cylinder 3. On the other side, the lip 253 cooperates with the cylinder 
237 as a check valve when a pressure is conveyed through the connection 
256 that dominates the pressure present in the secondary chamber 246. 
The said displacement of the separating piston 242 is generated by an 
auxiliary pressure from the auxiliary pressure source 57. The auxiliary 
pressure line 56 leading from the auxiliary pressure source 57 is guided 
past an auxiliary pressure connection 258. The auxiliary pressure 
connection 258 is located in the lid 240, for example. 
A further lip sealing ring 259 is located between lip sealing rings 249 and 
251, inside a circumferential groove 260 cut into the separating piston 
242. The lip sealing ring 259 has a sealing lip 261 which, like the 
sealing lip 253, is oriented toward the secondary chamber 246. The 
distance of the sealing lip 261 from the sealing lip 253 is at least as 
great as the longest possible stroke of the separating piston 242 inside 
the cylinder 237 starting from the lid 240, in the direction toward the 
end wall 239. A further valve opening 262 is associated with the lip 
sealing ring 259, and the two together form a 2/2-way valve. The valve 
opening 262 originates from a connection 263. The connection 263 is 
connected via at least one line segment 264 to the reservoir 4 of the main 
brake cylinder 3. In the base position of the separating piston 242, the 
valve opening 262 is open, so that an annular space 265 located between 
the lip sealing rings 251 and 259 communicates with the reservoir 4. When 
the separating piston 242 is displaced counter to the force of the 
separating piston restoring spring 247, starting at the lid 240 and with 
the use of auxiliary force introduced through the connection 258, the 
sealing lip 261 passes across the valve opening 262, so that an 
overpressure that is possibly present in the annular space 265 is 
maintained relative to the pressure in the reservoir 4. 
A further annular space 266 is located between the lip sealing ring 259 and 
the lip sealing ring 249. When the auxiliary motor 50 is shut off, the 
auxiliary pressure line 56 is unpressurized. As a result, the separating 
piston 242 rests against the lid 240 in the position shown, which is the 
normal position. Consequently, the above-mentioned passage is made 
possible from the main brake cylinder 3 through the secondary chamber 256 
and the connection 257 to the anti-skid system 11. The pressure generated 
in the main brake cylinder 3 by means of the operation of the brake pedal 
6 can also be transmitted through the anti-skid system 11 in the direction 
of the front-wheel brakes 7 and 8. Of course, the same applies for the 
rear-wheel brakes 9 and 10, because the cylinder 238 installed between the 
main brake cylinder 3 and the anti-skid system 11 is embodied in the same 
manner described above. 
The connection 257 of the cylinder 237 communicates with the brake pressure 
buildup valves 19 of the brake pressure modulation valve arrangements 15 
and 16 via the normally open reversing valve 39 of the brake circuit I. A 
bypass that can be opened in the direction toward the said brake pressure 
modulation valve arrangements 15 and 16 by means of the check valve 43 is 
disposed in the direction from the cylinder 37 to the brake pressure 
modulation valve arrangements 15 and 16. The reversing valve 39 is 
preferably combined with the pressure-limiting valve 41 so that, with 
electrical activation, instead of a free passage, a passage in the 
direction toward the main brake cylinder 3 only results when a sufficient 
drop in pressure predominates between the brake pressure buildup valves 19 
and the main brake cylinder 3. This is of such a magnitude that when the 
main brake cylinder 3 is not operated, the pressure present in front of 
the pressure-limiting valve 41 is sufficient for automatic braking and, if 
necessary, blockage of wheels. 
The reversing valve 40, the pressure-limiting valve 42 and the check valve 
44 are installed in the same manner between the cylinder 238 and the brake 
pressure buildup valves 19 of the brake pressure modulation valve 
arrangements 17 and 18. 
The 2/2-way valves 45 and 46 are closed in their normal positions, and can 
be opened electrically. These 2/2-way valves 45 or 46 are located between 
the connections 257 of the cylinders 237 or 238 and inlets 89, 90 of the 
recirculating pumps 13 or 14. For the purpose of decoupling the secondary 
chambers 247 of the cylinders 237 or 238 from the storage chambers 21 or 
22, the said check valves 91 or 92 are disposed between these and the 
2/2-way valves 45 or 46. These check valves 91 and 92 can be considered to 
be part of the device 12, because the execution of anti-locking operation 
alone renders these check valves 91 and 92 unnecessary with the use of 
so-called free-piston pumps as recirculating pumps 13, 14. 
A black box 93 is associated with the additional device 12. The black box 
93 includes means such as those described in detail in the introduction to 
the description for monitoring the vehicle for the start of the danger of 
skidding. Therefore only the key terms are mentioned here, such as 
gyroscope and monitoring of the angular acceleration, acceleration 
sensors, expanded use of wheel revolution sensors and wheel-skid measuring 
devices and threshold value switches for obtaining control signals that 
act on the control circuit 30 and, via this control circuit, control the 
auxiliary pressure source 57, the recirculating pumps 13 and 14, and the 
brake pressure modulation valve arrangements 15 through 18, the reversing 
valves 39 and 40 and the 2/2-way valves 45 and 46. 
If a danger of skidding occurs during an unbraked execution of a curve in a 
vehicle equipped in accordance with the invention, for example, this is 
detected by the monitoring device outlined with the black box 93 that, 
when a predetermined limit value has been exceeded, controls the brake 
pressure buildup valves 19 of the rear-wheel brakes, activates the 
auxiliary motor 50, opens the 2/2-way valve 45, closes the reversing valve 
39 and activates the motor 27 for driving the recirculating pump 13, for 
example. By means of this, the auxiliary pump 49 generates an auxiliary 
pressure that travels through the line 56 into the primary chamber 245 of 
the cylinder 237 and displaces the separating piston 242. In the process, 
the separating piston 242 closes the 2/2-way valve 255 formed by the lip 
sealing ring 251 and the valve opening 254, and a pressure is generated in 
the secondary chamber 246, so that pressure medium flows out of this 
secondary chamber 246, through the opened 2/2-way valve 255 to the inlet 
89 of the recirculating pump 13, and through the check valve 43 to the 
wheel brake cylinders 7 and 8. In this way, the recirculating pump 13 is 
indirectly filled by means of auxiliary pressure supplied into the 
cylinder 237 so that the pump can build up a pressure ahead of the closed 
reversing valve 39 that travels through the open brake pressure buildup 
valves 19 of the brake pressure modulation valve arrangements 15 and 16 
and into the wheel brakes 7 and 8. Both front wheels are braked in this 
case, for example, because of which the lateral guiding forces of the 
front wheels are reduced, as the prior art teaches. This has the desired 
effect that lateral skidding results or is increased to a magnitude or 
possibly beyond the magnitude of the lateral skidding of the rear wheels.. 
This has the intended effect that an increase in the rotating velocity of 
the vehicle around the vertical axis, that is, skidding, is counteracted, 
or skidding is halted. 
When an intended braking pressure in the above sense is attained, the brake 
pressure buildup valves 19 of the brake pressure modulation valve 
arrangements 15 and 16 are closed, by means of which at least initially 
the braking pressures in the wheel brakes 7 and 8 remain constant. 
Because a surplus of pressure medium conveyed from the recirculating pump 
through the pressure-limiting valve 41 during such a period of time would 
cause a useless energy consumption and disturbing pumping noise,the 
2/2-way valve 45 can be temporarily closed by means of the control device 
30 during such a period of time. Because of this, the inlet 89 of the 
recirculating pump 13 receives no pressure medium, and the recirculating 
pump runs empty. 
If the means disposed inside the black box 93 detect that a skidding danger 
has been sufficiently counteracted, that is, that a fall below a 
preselected threshold value has occurred, the braking pressure first 
contained in the wheel brakes 7 and 8 can be at least partly reduced. This 
is achieved by the opening of the braking pressure reduction valves 20 of 
the brake pressure modulation valve arrangements 15 and 16. By means of 
this, pressure medium travels out of the wheel brakes 7 and 8 to the 
storage chamber 21, and finally also through the check valve 91 into the 
recirculating pump, from where it is forced back through the 
pressure-limiting valve 41 into the cylinder 237. In the process, the 
separating piston 242 recedes counter to the low auxiliary pressure set by 
the auxiliary pressure-limiting valve 51. 
When a new pressure buildup is required, the 2/2-way valve 45 is opened, by 
means of which the recirculating pump 13 can build up pressure again that 
can be conveyed to the wheel brakes 7 and 8 in the above-described manner, 
by means of the brake pressure buildup valves 19. 
In the described case, only wheel brakes 7 and 8 are used to improve 
cornering of the vehicle. For the previously described case, the premise 
was the danger of skidding by means of drifting of the rear axle, i.e., a 
so-called oversteering tendency. Vehicles with an oversteering tendency 
are widespread, but there are also vehicles with an understeering 
tendency. Depending on the type of load, it can even occur that a vehicle 
has an oversteering tendency at one time and an understeering tendency at 
another. If the understeering tendency occurs, the dynamics resulting from 
inadequate lateral guiding force of the front wheels can be counteracted 
in a compensating manner in that lateral guiding forces of the rear wheels 
are automatically reduced. For this purpose braking forces should be 
automatically supplied to the rear-wheel brakes 9 and 10. This occurs in 
the same sense as described for the wheel brakes 7 and 8 of the front 
wheels. Thus, another description that would only be distinguished from 
the preceding one by different reference numerals is superfluous. 
The danger of skidding can arise not only when driving at an essentially 
uniform speed, but it can also be triggered by means of braking during 
cornering. This occurs in particular because a dynamic increase in the 
load on the front wheels and a dynamic reduction in the load on the rear 
wheels arise during braking, although the center of gravity of the vehicle 
retains its orientation relative to the front axle and the rear axle. 
Lateral guiding forces of the rear wheels clearly tend to be weaker 
relative to the lateral guiding forces of the front wheels, so that an 
angular acceleration of the vehicle around its center of gravity can be 
set in, and an unacceptable limit value can be attained. In this case it 
is necessary to automatically increase a braking pressure selected for the 
front wheel brakes 7 and 8 by the driver, by operating the brake pedal 6. 
This is also possible through the startup of the additional device 12 and 
the anti-skid system 11. With this startup the combination of the 
anti-skid system 11 and the additional device 12 for automatic braking 
dominates the driver's judgment. If the driver has already braked, the 
automatic brake operation causes an increased braking pressure, at least 
for one of the front-wheel brakes, so that the sum of the lateral guiding 
forces of the front wheels is reduced, as with the prior art described in 
the introduction. 
Because it can be seen that the vehicle braking system is in the position 
to automatically inject braking pressures: individually into all wheel 
brakes of four vehicle wheels, thanks to the four brake pressure 
modulation valve arrangements 15 through 18, this vehicle braking system 
is also suited for executing traction control, regardless of whether the 
front wheels or the rear wheels, or the front and rear wheels are powered. 
Traction control is executed in accordance with the known requirements, 
again with the use of the auxiliary pump 49 and the cylinders 237 and 238, 
as well as the reversing valves 39, 40, the 2/2-way valves 45 and 46 and 
the anti-skid system 11 in the case of traction control at all four 
wheels. Because traction control devices are noted for drivable front 
wheels, to which are assigned diagonal brake circuits, and also for rear 
wheels, for the majority of which a front-back brake circuit division is 
selected, a description of control or function for traction control is 
superfluous. 
Depending on the situation, it can become necessary during traction control 
operation to further increase the braking pressure present in this case in 
the respective wheel brakes by means of operating the brake pedal. This 
can be achieved by means of hydraulically overcoming the force of 
adherence of the lips 253 against the cylinders 237 or 238. Because of 
this, a flow around the lip sealing ring 251 is possible. If, at this 
point in time, the reversing valve 39 of the front-wheel brakes, which 
have wheels that can be powered as a component should still be closed, the 
pressure opens the check valve 43, which forms a bypass around the still 
impassable reversing valve 39, so that an increase in braking pressure in 
the wheel brakes 7 and 8 is possible in the way desired by the driver, and 
virtually without delay. Although during the course of this, the 
separating piston 242, because it is charged with a pressure from the 
secondary chamber 246 that is higher than the auxiliary pressure in the 
primary chamber 245, will yield in the direction of its initial position 
with respect to the lid 240, this does not cause an interfering 
disadvantage, because previously the wheel brakes 7 and 8 were filled by 
the displacement of the separating piston 242 by only the auxiliary 
pressure. 
FIG. 1 is provided with reference numerals for front-wheel brakes and 
rear-wheel brakes in such a manner that the brake circuit I is a so-called 
front-axle brake circuit and brake circuit II is a so-called rear-axle 
brake circuit. A brake circuit division of this type is also characterized 
as a TT brake circuit division. The concept of the invention is not 
limited to such TT brake circuits, however, because it is apparent that 
so-called diagonal brake circuits, characterized as K brake circuits, can 
be achieved by means of switching the reference numerals of front- and 
rear-wheel brakes. In this case all four wheels can be automatically 
supplied individually with braking pressures, thus counteracting a 
disadvantageous dynamic individual behavior of the vehicle. 
As already mentioned in the introduction to the description, during a 
normal braking procedure controlled by means of the brake pedal 6, leakage 
of the lip sealing ring 251 relative to the cylinder 237 or 238 has the 
effect, because the separating piston 242 is in its base position, that 
the brake pedal 6 goes further down than would normally correspond to the 
intended braking pressure. In the process, pressure medium draining 
between the lip sealing ring 251 and the cylinder 237 and into the annular 
chamber 265 flows through the valve opening 262, the connection 263 and 
the at least one line segment 264, into the reservoir 4. There the 
pressure medium is then available prior to a new operation of the brake 
pedal 6 for the main brake cylinder 3. Leakage can thus be detected 
without pressure medium escaping from the hydraulic vehicle brake system 
into the environment. Because of this, it is possible to use the brakes as 
often as desired before reaching the next service station. A certain 
safety risk, therefore, only lies in the fact that the brake pedal must be 
released intermittently during deliberate vehicle deceleration for 
so-called re-pumping, because during this release there is, of course, no 
braking effect. 
During operation of the hydraulic vehicle brake system, the lip sealing 
ring 259 is stressed less than the lip sealing ring 251. This has the 
advantage that a defective lip sealing ring 251 can be identified 
relatively early on in normal braking operation. 
Should the sealing ring 240 on the side of the primary chamber wear out and 
therefore leak, pressure medium trickling through it, coming from the 
auxiliary pressure source 57, can flow along it through the annular 
chamber 266 and again through the connection 263 and the at least one line 
segment 264 and into the reservoir 4, so that this pressure medium is 
available for re-use. Should this pressure medium contain air bubbles or 
gas bubbles for some reason, these will also flow off in the direction of 
the reservoir 4, because a flowover into the secondary chamber 246 is 
counteracted by the displaced lip sealing ring 259 on the one hand or, 
provided that this ring is not displaced, by the lip sealing ring 251 on 
the other hand, provided that it is tight. 
The foregoing relates to a preferred exemplary embodiment of the invention, 
it being understood that other variants and embodiments thereof are 
possible within the spirit and scope of the invention, the latter being 
defined by the appended claims.