Abstract:
In a hydraulic vehicle braking systems operable via hydraulic external energy from a pump or, in the event the pump malfunctions, via hydraulic energy made available using a master brake cylinder via muscle-powered energy, provision is made for safe braking via muscle-powered energy such that at least one cylinder-piston configuration isolates at least two wheel brakes from the hydraulic pressure medium that is able to be pumped by the pump. The hydraulic vehicle-braking system incorporates wheel brakes of one axle that is isolated from a hydraulic pump in terms of pressure medium by a cylinder-piston configuration, which wheel brakes are supplied with pressure medium by one master cylinder chamber of a master brake cylinder. The other master brake cylinder chamber is allocated to the two other wheel brakes of the other vehicle axle, so that given bubble-free pressure medium downstream from the pump, the wheel brakes of both vehicle axles make a contribution to vehicle deceleration.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a hydraulic vehicle braking system.  
         BACKGROUND INFORMATION  
         [0002]    Known from published patent documents DE 196 36 432 A1 and WO 98/31576 are hydraulic vehicle braking systems having wheel brakes for four wheels distributed to a first and a second vehicle axle; a hydraulic external energy source; electrically controllable service brake valve configurations situated between the external energy source and the wheel brakes; a braking value indicator operable by a brake pedal for performing service braking via hydraulic external energy using the service brake valve configurations; a dual-circuit master brake cylinder that is operable via the brake pedal and has a first master brake cylinder chamber and a second master brake cylinder chamber for performing auxiliary braking via muscle-powered energy, a normally open operation mode switching valve that is controllable for the service braking operation in a closed position being situated in each brake circuit between the master brake cylinder and the wheel brakes being able to be supplied with pressure medium by this master brake cylinder, and at least the service brake valve configurations being controllable via a control and regulating device. In this context, the wheel brakes of a front vehicle axle belong to a first brake circuit, and the wheel brakes of a back vehicle axle are assigned to the second brake circuit. In the case of a power outage and a thus inoperable control and regulating device, the electrically controllable service brake valve configurations are not able to be put into operation, and the four wheel brakes of the two vehicle axles are connected through the normally open operation mode switching valves to the dual-circuit master brake cylinder. As a result, all four wheel brakes and, consequently, all wheels of the vehicle are able to be braked by muscle force on the brake pedal. In this context, starting from a vehicle to be equipped and from load examples to be assumed for this vehicle, one skilled in the art may determine a distribution of the braking force to the front wheel brakes and the back wheel brakes by selecting the dimensions of the brake components. The control and regulating device may be set up, for example, according to the SAE Technical Papers Series No. 960991 entitled “Electrohydraulic Brake System The First Approach to a Brake-by-Wire-Technology,” page 111, under the heading “Safety Concept,” such that, in response to the failure of a service brake valve configuration, the control and regulating device determines whether using external energy for three wheels results in a shorter braking path or using muscle-powered energy for four wheels results in a shorter braking path in order after such a determination to control those valves resulting in the shorter braking path or to leave them alone. In this context it should be mentioned that on icy or snowy roads, for example, it may be better to brake four wheels using muscle-powered energy, thereby making the vehicle more easily controllable than in the case of asymmetrical braking using only three wheel brakes and operation via external energy. It cannot be completely ruled out that in the case of operation via external energy, air or gas dissolved in the hydraulic pressure medium results in the formation of air or gas bubbles due to a pressure drop in the hydraulic pressure medium. Air or gas bubbles in the hydraulic pressure medium make it compressible so that in the case of a sudden transition from the service braking operation via external energy to the auxiliary braking operation via muscle-powered energy, the disadvantage may arise that the air or gas bubbles are to be compressed via muscle-powered energy using the master brake cylinder. Since the master brake cylinder has a limited pump capacity for each brake pedal travel, it cannot be ruled out that as a result of the elasticity of the air or gas bubbles, the wheel brake pressure does not increase as much as desired for decelerating the vehicle.  
           [0003]    A hydraulic vehicle braking system known from published patent document WO 98/28174 has wheel brakes for four wheels distributed to a first and a second vehicle axle; a hydraulic external energy source; electrically controllable service brake valve configurations situated between the external energy source and the wheel brakes; a braking value indicator operable by a brake pedal for performing service braking via hydraulic external energy using the service brake valve configurations; and a dual-circuit master brake cylinder operable by the brake pedal for performing auxiliary braking using muscle-powered energy, where a normally open operation mode switching valve that is able to be controlled for the service braking operation in a closed position is situated in each brake circuit between the master brake cylinder and the wheel brakes able to be supplied with pressure medium by this master brake cylinder; at least the service brake valve configurations are controllable by a control and regulating device; a cylinder-piston configuration is installed in each case between the wheel brakes of those vehicle axles designed for a greater contribution to the vehicle braking and the corresponding electrically controllable service brake valve configurations; and the wheel brakes designed for a greater contribution to the vehicle braking are connected to the operation mode switching valves associated with the two brake circuits of the master brake cylinder, has the advantage that the wheel brakes of this vehicle axle and the dual-circuit master brake cylinder are hydraulically isolated from the pressure medium that is able to be placed under pressure by a pump for the service braking operation and is able to be stored in pressure accumulator and, therefore, may contain air and/or gas in the form of bubbles, for example due to a pressure drop. In this respect, the combination of master brake cylinder and the two corresponding wheel brakes isolated from the pressure medium able to be pumped by the pump has the same degree of reliability as a conventional hydraulic vehicle braking system. Since only the wheel brakes of one vehicle axle are able to be braked via the pressure medium from the master brake cylinder, the vehicle axle that is typically the more loaded vehicle axle is selected for braking using the pressure medium from the master brake cylinder.  
           [0004]    U.S. Pat. No. 5,941,608 discloses another hydraulic vehicle braking system that is set up, on the one hand, for braking four vehicle wheels via external energy using service brake valve configurations and, on the other hand, for braking via muscle-powered energy using a dual-circuit master brake cylinder. In this context, four cylinder-piston configurations are provided for isolating the four wheel brakes from the pressure medium that is able to be placed under pressure by a pump and is able to be controlled by the service brake valve configurations to direct against the individual pistons of the cylinder-piston configuration. As a result, braking pressure is able to be reliably generated in the four wheel brakes via the master brake cylinder and via the pressure medium contained therein. The necessary technical outlay for this is noticeably higher than for a hydraulic braking system according to U.S. Pat. No. 5,941,608.  
         SUMMARY OF THE INVENTION  
         [0005]    The hydraulic vehicle braking system of the present invention is less expensive than the hydraulic vehicle braking system according to U.S. Pat. No. 5,941,608, and provided that no gas and/or air bubbles are present in the vehicle braking system, renders it nevertheless possible to brake via the wheel brakes of the vehicle axle contributing less to the vehicle deceleration, so that improved vehicle deceleration with typically better controllability of the vehicle is achieved in comparison to the hydraulic vehicle braking system according to document WO 98/28174.  
           [0006]    One advantage of this invention is that, as a result of an individual operation mode switching valve being present for each wheel brake cylinder of a brake circuit, only one wheel brake is isolated from the master brake cylinder in the case of a technical malfunction, for example, and no longer contributes to braking. In addition, the same advantage may be provided for the vehicle axle whose wheel brakes contribute less to the total vehicle deceleration in the normal case. Another feature of the present invention results in the possibility of supplying two wheel brakes of a vehicle axle with pressure medium from the master brake cylinder via only one operation mode switching valve. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 shows a hydraulic circuit diagram for a first exemplary embodiment.  
         [0008]    [0008]FIG. 2 shows a hydraulic circuit diagram for a second exemplary embodiment.  
         [0009]    [0009]FIG. 3 shows a hydraulic circuit diagram for a third exemplary embodiment of the hydraulic vehicle braking system. 
     
    
     DETAILED DESCRIPTION  
       [0010]    Hydraulic vehicle braking system  2  as shown in FIG. 1 has four wheel brakes  3 ,  4 ,  5 , and  6 , which are distributed on a first and a second vehicle axle (not shown); a hydraulic external energy source, which includes a supply tank  7 , a pump  9  driven by an electromotor  8 , a pressure accumulator  10 , a pressure sensor  11 , and a safety valve  12 ; electrically controllable service brake valve configurations  13  through  20 , which are situated between this external energy source and wheel brakes  3  through  6 ; a dual-circuit master brake cylinder  22 , which is operable via a brake pedal  21  and has a first master brake cylinder chamber  23  and a second master brake cylinder chamber  24  for powering wheel brakes  3  through  6 ; a normally open operation mode switching valve  25  and  26  between second master brake cylinder chamber  24  and wheel brakes  3  and  4 , respectively; another normally open operation mode switching valve  27  and  28  between first master brake cylinder chamber  23  and wheel brakes  5  and  6 , respectively; and a control and regulating device  29 . Furthermore, cylinder-piston configurations  30 ,  31  are allocated to the wheel brakes of the vehicle axle (not shown) that makes a greater contribution to the vehicle braking. These each have a cylinder  32 , a piston  33  movable therein, and a return spring  34  for piston  33 . The appropriate cylinder  32  is connected to wheel brake  3  and  4 , respectively, where return spring  34  supports itself in cylinder  32 . The appropriate cylinder  32  is connected to a corresponding service brake valve configuration  13 ,  14 , and  15 ,  16 , respectively, where piston  33  assumes its initial position due to return spring  34 . For example, wheel brakes  3  and  4  belong to the front axle of a passenger vehicle. In the case of a passenger vehicle, such a front axle is normally the axle designed for a greater contribution to vehicle braking.  
         [0011]    Service brake valve configurations  13 ,  14  and  15 ,  16 , respectively, are used, on the one hand, to lead pressure medium from pump  9  or pressure accumulator  10  into cylinder-piston configuration  30  and, on the other hand, to lead pressure medium from this cylinder-piston configuration  30  back to pressureless supply tank  7 . In this context, service brake valve configuration  13  is represented as a two-connection valve of the directional control valve type that may be brought into its closed position by a spring  35  and may be opened to be more or less wide via an electromagnet  36 . Service brake valve configuration  13  is often also referred to as a brake pressure build-up valve. In this instance, service brake valve configuration  14 , which is assigned to same wheel brake  3 , is also designed as a two-connection valve whose spring  37  may open service brake valve configuration  14  and whose electromagnet  38  maybe used to partially or completely close service brake valve configuration  14 . Service brake valve configurations  13  and  14 , which are each represented in the form of two squares associated with one another, are assigned parallel marks (not shown) that according to the ISO  1219  standard are the symbols for continuous adjustability between 0% and 100% of a constructively provided opening cross-section. It is inferable from FIG. 1 that service brake valve configurations  15 ,  17 , and  19  have the same design as previously described with reference to service brake valve configuration  13 . Furthermore, it is also inferable from FIG. 1 that service brake valve configuration  16  has the same design as previously described service brake valve configuration  14 . Service brake valve configurations  18  and  20  have the same design as service brake valve configurations  17  and  19  and are, therefore, normally closed. In addition, reference is made to the fact that one skilled in the art may replace pairs of service brake valve configurations  13  and  14 ,  15  and  16 , etc., with valves known in the field of vehicle braking systems, having three connections and three valve positions, in which case instead of two electromagnets  36  and  38 , a single electromagnet able to be powered by different current intensities is sufficient. Furthermore, reference is also made to the fact that so-called pressure-regulating valves symbolized by a single square may also be installed instead of service brake valve configurations  13  through  20 , which arc symbolized in each case by a pair of squares. Pressure differences between pump  9  or pressure accumulator  10  and corresponding cylinder  32  on the one hand and between this corresponding cylinder  32  and pressureless supply tank  7  on the other hand may be adjusted via such pressure-regulating valves. For the purpose of supplying current, electromagnets  36  and  38  of service brake valve configurations  13  through  20  are connected to control and regulating device  29 , this being represented for service brake valve configurations  20  and  13  by sketched electric lines.  
         [0012]    As a result of wheel brakes  3  and  4  being allocated to a front axle contributing significantly to vehicle deceleration, wheel brakes  5  and  6  are back wheel brakes at a back axle (not shown) of the vehicle. These back wheel brakes  5  and  6  may be supplied via service brake valve configurations  17  and  19 , respectively, with pressure medium from pump  9  or pressure accumulator  10  and may be connected via service brake valve configurations  18 ,  20  with pressureless supply tank  7 , so that brake pressures that are achieved dissipate in wheel brakes  5  and  6  and may also be removed.  
         [0013]    In this context, service brake valve configurations  13  through  20  of the “continuous valve” type shown in the exemplary embodiment as shown in FIG. 1 have the advantage that pressure medium flows to wheel brakes  3  through  6  or away from wheel brakes  3  through  6  which maybe constantly accelerated or decelerated. This is advantageous when adjusting brake pressures in wheel brakes  3  through  6  and also has a noise-lowering effect. In contrast, operation mode switching valves  25 ,  26 ,  27 , and  28  are configured in a simple manner because they are not intended for changing the wheel brake pressures.  
         [0014]    So that the previously mentioned adjusting of brake pressures in wheel brakes  3 ,  4 ,  5 , and  6  is possible, an individual brake pressure sensor  39  is assigned to each wheel brake  3  through  6  in a manner known per se. As indicated for brake pressure sensor  39  of wheel brake  6 , these brake pressure sensors  39  are connected via electric lines (not shown) to control and regulating device  29 , so that wheel brake pressures present in wheel brakes  3 ,  4 ,  5 , and  6  are available as actual values for evaluating control and regulating device  29 .  
         [0015]    An additional pressure sensor  40  is connected to first master brake cylinder chamber  23 . This pressure sensor  40  is connected to control and regulating device  29  in the indicated manner. In addition and, in case of emergency, for redundancy, a displacement sensor  41 , which is indirectly adjustable via brake pedal  21 , is situated near master brake cylinder  22 . In addition, a so-called brake lights switch  42  may be arranged near master cylinder  22  and may also be controlled via brake pedal  21 . Displacement sensor  41  as well as brake lights switch  42  are connected to control and regulating device  29 , so that it is able to receive signals from displacement sensor  41  and brake lights switch  42 . The combination of brake pedal  21  and both sensors  40  and  41  is referred to as the braking value indicator. For example, short pedal paths are determined by displacement sensor  41  and large forces on brake pedal  21  are indirectly measured by pressure sensor  40 .  
         [0016]    If a power supply device (not shown) properly powers control and regulating device  29  and the latter is in order, a pressure variable signal from pressure sensor  11  is monitored by control and regulating device  29  and the electromotor, for example, is switched on as a function of the variable of the pressure signal in order to replenish pressure accumulator  10  via pump  9  with pressure medium from supply tank  7 . When a preplanned pressure level is reached, control and regulating device  29  switches off electromotor  8 . As a result, the hydraulic external energy source, including pressure accumulator  10 , is available for braking actions using service brake valve configurations  13  through  20 .  
         [0017]    In response to brake pedal  21  being operated and brake lights switch  42  being consequently closed, the availability of the hydraulic external energy source detectable via pressure sensor  11  causes operation mode switching valves  25 ,  26 ,  27 , and  28  to be electromagnetically brought into their blocking positions. As a result, master brake cylinder chambers  23  and  24  are hydraulically isolated from wheel brakes  3  through  6 . In response to a further operation of brake pedal  21  and, in this context, with a cylinder-piston-spring configuration  43 , which communicates with master brake cylinder chamber  23  and is, for example, of a type selectable from the related art referred to e.g. as a brake actuation simulator, pressure sensor  40  transmits a pressure signal and displacement sensor  41  transmits a displacement signal to control and regulating device  29 . In a manner known in the art, this control and regulating device  29  is set up such that, in the case of a present pressure sensor signal and displacement sensor signal, the control and regulating device detects an initial zero wheel brake pressure display from brake pressure sensors  39  as a deviation and, in accordance with the deviation, controls service brake valve configurations  13 ,  15 ,  17 , and  19 , which are intended for building up brake pressure in the wheel brakes, in the sense of an at least partial opening. Since in this context brake pressure valve configurations  14 ,  16  are closed and operation mode switching valves  25  through  28  are controlled into their closed positions, all of the pressure medium flowing through service brake valve configurations  13 ,  15 ,  17 , and  19  to wheel brakes  3  through  6  leads to increases in brake pressure. In this context, feedback is continuously provided via wheel brake pressure sensor  39  to control and regulating device  29 , so that control and regulating device  29  detects the wheel brake pressure values approaching such values predefined by a driver operating the brake pedal, using pressure sensor  40  and/or displacement sensor  41 . Of course, using pressure displays from pressure sensor  40  by means of software and a computer, control and regulating device  29  may be set up to calculate wheel brake pressures for front wheel brakes  3  and  4 , which deviate from the wheel brake pressures provided for back wheel brakes  5  and  6  in the sense of a braking force distribution of a total braking force to front wheel brakes and back wheel brakes. Such a development of control and regulating device  29  may be desired since very different load possibilities may result in a vehicle having very different and possibly also very high locations of center of gravity.  
         [0018]    In addition, reference is also made to the fact that control and regulating device  29  is able via wheel rotation sensors (not shown) and also via a yaw rate sensor (not shown), for example, to detect overbraking of the wheels or bad tracking when driving and, in compliance with the control algorithm derivable from the related art, is accordingly able to provide for brake-slip reduction or also for corrective yawing moments using asymmetrical braking. However, since brake-slip regulation and yaw moment generation supporting the driver are part of the related art as already indicated, it is not necessary to go into further detail here.  
         [0019]    If during operation of a vehicle equipped with this hydraulic vehicle braking system the power supply of control and regulating device  29  fails for example, service brake valve configurations  13  through  20  are no longer controllable and remain in the basic position shown in FIG. 1 or return to it. A lack of current for control and regulating device  29  results in operation mode switching valves  25  through  28  assuming their basic position and returning to it. As a result, as may be seen from FIG. 1, that master brake cylinder chamber  24  communicates with wheel brakes  3  and  4 , and master brake cylinder chamber  23  communicates with wheel brakes  5  and  6 . Therefore, as a result of operating brake pedal  21  pressure medium is able to be pressed from both master brake cylinder chambers  23  and  24  to wheel brakes  3  and  4  as well as, wheel brakes  5  and  6 , so that vehicle decelerations are only possible using muscle-powered energy.  
         [0020]    It is first assumed that there are no air bubbles and no gas bubbles between pressureless supply tank  7  and service brake valve configurations  13 ,  15 ,  17 , and  19 , and moreover that no air bubbles and no gas bubbles are in wheel brakes  5  and  6 . In any case, as a result of cylinder-piston configurations  30  and  31  known from the related art, the pressure medium in wheel brakes  3  and  4  and in master brake cylinder chamber  24  is free of gas bubbles and air bubbles. Therefore, in response to brake pedal  21  being actuated, all of the muscle-powered energy supplied to brake pedal  21  results in wheel brake pressures in all wheel brakes  3  through  6  for decelerating the vehicle.  
         [0021]    If gas bubbles or air bubbles are contained in wheel brakes  5  and  6 , which are able to contribute less than wheel brakes  3  and  4  to the deceleration of the vehicle, less brake pressure would be achieved than in wheel brakes  3  and  4  depending on the volume of gas or air contained in wheel brakes  5  and  6 . This is a disadvantage generally only to be expected as an exception. However, this is contrasted by the advantage that, as already mentioned in the introductory part of the specification, those wheel brakes contributing the most to vehicle declaration are able to be used with great reliability, and that two further wheel brakes are able to help with vehicle deceleration even during the muscle-powered braking operation. This is particularly advantageous on slippery ice since front wheels, e.g., of passenger vehicles, have a tendency to lock up significantly earlier on slippery ice than back wheels when they are braked by conventional braking systems, for example. As already mentioned, this results in a deceleration gain in comparison with a hydraulic vehicle braking system described in WO 98/28174. Since this previously known vehicle braking system satisfies legal regulations, an improvement as a result of the embodiment of a hydraulic vehicle braking system as shown with reference to FIG. 1 is achievable in a particularly inexpensive manner.  
         [0022]    Since in the exemplary embodiment according to FIG. 1 an individual operation mode switching valve  25 ,  26 ,  27 , and  28 , is situated between every wheel brake  3  through  6 , respectively, and a corresponding master brake cylinder chamber  23  and  24 , it is possible for example when one of service brake valves  13  or  15  or  17  or  19 , which build up or increase the brake pressure, is no longer able to be brought into its open position, to bring one of operation mode switching valves  25  through  28  into its open position, so that, for example, three of the wheel brakes arc able to be operated by hydraulic external energy and the remaining wheel brake is able to be supplied with pressure from one of master brake cylinder chambers  23  or  24 . It is evident that there is only a partial failure of braking force at one wheel brake. It is clear that this results in only an insignificant reduction in vehicle deceleration, and the vehicle driver is able to compensate for a simultaneously generated yaw moment by steering or by stability regulation.  
         [0023]    The second exemplary embodiment of a hydraulic vehicle braking system  2   a  according to FIG. 2 differs from the exemplary embodiment according to FIG. 1 in that a normally open brake pressure compensation valve  50  of an electrically controllable type is situated between two back wheel brakes  5  and  6 , and operation mode switching valve  28  according to FIG. 1 is dispensed with. Accordingly, in the braking operation per muscle-powered energy on brake pedal  21 , back wheel brake  5  on the one side is supplied with pressure medium for generating brake pressure and brake force as a result of open operation mode switching valve  27 , and as a result of open brake pressure compensation valve  50 , other back wheel brake  6  on the other side is also supplied with pressure medium for generating brake pressure and brake force. Since such a brake pressure compensation valve  50  between the wheel brakes of one axle, of a back wheel axle (not shown) in this instance, is known in the related art, e.g., according to document WO 98/28174, a description of the advantages is not necessary here. During normal service braking operation using hydraulic external energy, i.e., when there is no danger of wheel lockup at any of the wheels of this axle, it is sufficient to operate service brake valve configuration  17  to supply brake pressure to wheel brakes  5  and  6 . After that, it is sufficient, e.g., to operate only service brake valve configuration  18  in order to lower a common brake pressure of wheel brakes  5  and  6  to a desired amount. In this context, pressure displays from one of the two brake pressure sensors  39  are used. However, if there is a danger of wheel lockup on one side, brake pressure compensation valve  50  is to be closed in a known manner, so that the brake pressure in a left wheel brake  5  or a right wheel brake  6  is changeable via brake pressure adjustment independently of the other wheel brake. For example, control and regulating device  29   a  is able to be set up such that, when service brake valve  17  is unable to open, service brake valve  19  is operated to adjust the brake pressures in wheel brakes  5  and  6 . If control and regulating device  29   a  detects that an attempted operation of the service brake valve configuration also does not result in a pressure build-up, it provides for operation mode switching valve  27 , which was previously closed for the service braking operation, to return to its open position, thereby making it possible to press pressure medium via muscle-powered energy on brake pedal  21  from master brake cylinder chamber  23  into wheel brakes  5  and  6  for the purpose of braking. It is recognizable that, due to brake pressure compensation valve  50 , service brake valve configurations  17  and  19  are redundant valves for service braking operation via hydraulic external energy. In this respect, the degree of reliability in the service brake mode is increased. If all service brake valve configurations  17 ,  18 ,  19 , and  20  of back wheel brakes  5  and  6  remain in their represented basic positions as a condition of a malfunction, the auxiliary braking operation is still able to be carried out via muscle-powered energy for these two back wheel brakes  5  and  6 .  
         [0024]    The third exemplary embodiment of a hydraulic vehicle braking system  2   b  according to FIG. 3 differs from the exemplary embodiment according to FIG. 2 in that an additional brake pressure compensation valve  51  is installed between both cylinder-piston configurations  30  and  31  and in this context, also between service brake valve configurations  13  and  15 , which, as already described, are used to increase the brake pressure in an appropriate wheel brake  3  and  4 , respectively. This additional brake pressure compensation valve  51  is also normally open, so that pistons  33  of both cylinder-piston configurations  30  and  31  are able to be pressurized equally. The recognizable result is that in service braking operation, and as long as there is still no danger of wheel lockup, a left wheel and a right wheel of a shared axle are braked equally and provide for an uninterrupted steerability of the vehicle. Of course for laterally unequal adjustment of wheel brake pressures, e.g., due to a danger of wheel lockup at only one wheel, brake pressure compensation valve  51  is to be closed.  
         [0025]    Until now it was assumed that cylinder-piston configurations  30  and  31  are associated with front wheel brakes  3  and  4  since it was indicated that they make a greater contribution to vehicle deceleration than back wheel brakes  5  and  6 , provided that these from wheel brakes correspond to an average passenger vehicle, for example. Since the hydraulic external force braking system is also set up for decelerating heavy vehicles, e.g., for braking trucks having dual tires in the back, reference is made to the fact that in such a case, cylinder-piston configurations  30  and  31  are allocated to the back wheels or their wheel brakes.  
         [0026]    In addition, reference is also made to the fact that depending on the construction of master brake cylinder  22 , wheel brakes  3  and  4  may be connected to master cylinder chamber  23  and wheel brakes  5  and  6  to master cylinder chamber  24 .