Abstract:
The invention relates to a hydraulic brake system and a method for controlling a hydraulic brake system for a land vehicle having a predetermined holding capacity for hydraulic fluid and at least one wheel brake. The invention is characterized by detecting if a current holding capacity of the brake system has increased in relation to the predetermined holding capacity and, if so, by feeding hydraulic fluid to the at least one wheel brake in a controlled manner and an amount corresponding at least to the increase in capacity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National stage of International Application No. PCT/EP2006/005494 filed Jun. 8, 2006, the disclosures of which are incorporated herein by reference, and which claimed priority to German Patent Application No. 10 2005 026 734.3 filed Jun. 9, 2005, the disclosures of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     This invention concerns, in general, brake systems for land vehicles, and in particular, compensation for operational states of such brake systems with reduced braking effect. 
     In the case of brake systems for land vehicles, it is known that their braking effect falls when they are very stressed. This is also called “fading”. The causes of it include the temperatures which occur with high stress (e.g. at high braking pressures), and which cause increased volume holding capacity for the hydraulic fluid of the brake system as a whole, and in particular of individual wheel brakes. In this way, for instance, it can happen that displacement reserves of individual components of a brake system (e.g. main cylinder, brake servo, brake pedal) are used up before a sufficient braking pressure can be generated. The result can be that the brake system cannot provide a braking effect which the driver wants. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is, in the case of hydraulic brake systems, to compensate at least partly for reduced braking effect (fading). 
     To achieve said object, this invention provides a brake system and a method according to the independent claims. 
     The hydraulic brake system according to the invention is intended for a land vehicle, and has a controller with inputs to receive input parameters which indicate operational states of the land vehicle, and outputs to output control signals for the brake position. The hydraulic brake system also includes at least one wheel brake and at least one hydraulic fluid reservoir, to store hydraulic fluid. To convey hydraulic fluid in the brake system in a controlled manner, a controllable pump, which is intended in particular to feed hydraulic fluid in a controlled manner to the at least one wheel brake, is provided. The hydraulic brake system has, in predetermined parts, regions and/or components, a predetermined holding capacity, holding power or volume holding for hydraulic fluid. The predetermined holding capacity can, for instance, be present in the at least one wheel brake, and/or hydraulic connections or lines, which feed hydraulic fluid to and from the at least one wheel brake. 
     Here, below and in particular in the claims, “set up” should be understood as meaning that the controller is implemented at least structurally to provide the technical functional features which are given in each case. Additionally, the controller can be programmed to provide the technical functional features which are given in each case, e.g. using software code or computer programs. It is thus intended that the functions of the controller are provided by a corresponding hardware structure (e.g. ASIC). The controller can also be implemented so that it has a generally usable hardware structure, which in association with appropriate programming (e.g. permanently implemented software code, computer program which is provided on a computer-readable storage medium, software code or computer program which can be downloaded for operation) provides the technical functional features of the controller. 
     According to the invention, it is provided that the controller can determine, from the input parameters, a current holding capacity of the brake system for hydraulic fluid. If it is established that the current holding capacity is raised compared with the predetermined holding capacity, i.e. the predetermined parts, regions and/or components of the brake system can hold more hydraulic fluid, according to the invention it is provided that the controller provides control signals, to operate the pump so that a quantity of hydraulic fluid corresponding at least to the capacity increase is fed to the at least one wheel brake. This feeding of hydraulic fluid can also be called “prefilling” or “topping up” the at least one wheel brake with hydraulic fluid. 
     In one embodiment, the at least one wheel brake is “predictively prefilled”. 
     In this case, it is provided that the controller determines the current holding capacity during an operational state of the brake system in which the brake system is not actuated, i.e. generates no braking effect. 
     It is also provided that the controller provides control signals for the pump in such a way that a suitable quantity of hydraulic fluid to compensate for the capacity increase is also fed to the at least one wheel brake during such an unactuated operational state of the brake system. In particular, it is provided that a first quantity of hydraulic fluid, corresponding essentially to the capacity increase, is fed to the at least one wheel brake. 
     Preferably, hydraulic fluid which is stored in a hydraulic fluid reservoir, which is used to supply a main cylinder of the brake system, is used in this embodiment. 
     The brake system can include a valve arrangement, which in a first operational state blocks a first hydraulic connection for feeding braking pressure which a driver generates to the at least one wheel brake, and in a second operational state makes the first hydraulic connection. In this case it is provided that the controller, if an increased holding capacity is determined, can produce control signals for the valve arrangement, to bring the valve arrangement into the first operational state. 
     In a third operational state, the valve arrangement can block the first hydraulic connection and also block a second hydraulic connection, which is provided between the input side of the pump and the hydraulic fluid reservoir. In this case it is provided that the controller can generate control signals for the valve arrangement, said control signals bringing the valve arrangement into the third operational state, and can also provide control signals for the pump, said control signals ending the operation of the pump if the first quantity of hydraulic fluid is fed. 
     Preferably, in the case of this embodiment, the control signals of the controller to operate the pump to feed hydraulic fluid to the at least one wheel brake are provided at an instant which depends on at least one of the input parameters. In particular, in this case it is provided that these control signals are provided at an instant following which actuation of the brake system, i.e. initiation of a braking process, is expected. 
     In another embodiment, the controller determines the current holding capacity during an operational state of the brake system in which the brake system is actuated, i.e. generates braking effect. 
     In contrast to the previous embodiment, this embodiment ensures not “predictive prefilling” but “topping up” the at least one wheel brake during a braking process. With this embodiment, vehicle safety is considerably increased, since brake fading is counteracted even in the extreme case that the available actuation displacement of the brake pedal, because of the increased volume holding capacity of the brake system, is (completely) used up, because the pedal itself or the pistons of the main cylinder have reached the limit stop. 
     In this case it is provided that the controller can provide control signals for the pump, to feed a suitable quantity of hydraulic fluid for holding capacity compensation to the at least one wheel brake during such an actuated operational state of the brake system. In particular, it is provided that a second quantity of hydraulic fluid should be fed to the at least one wheel brake, said quantity of hydraulic fluid leading to an operational state of the at least one wheel brake in which anti-lock regulation is necessary to prevent locking or tendency to lock of the wheel brake(s) or associated wheels. 
     This embodiment too can include the above-mentioned valve arrangement with first and second operational states. However, it is provided here that if the valve arrangement has the second operational state, the controller can provide, for the pump, control signals which cause the second hydraulic fluid quantity to be fed to the at least one wheel brake. 
     Preferably, the second hydraulic fluid quantity is fed out of a hydraulic fluid reservoir which is a low pressure hydraulic fluid reservoir of the brake system, and is used to hold hydraulic fluid which flows out of wheel brakes during a pressure reduction phase of anti-lock regulation. 
     Preferably, the brake system is controlled so that the second hydraulic fluid quantity, which results in an operational state which requires anti-lock regulation, is fed to one or more wheel brakes, which is or are provided for a front axle or front wheels of the land vehicle. 
     Preferably, the controller determines the current holding capacity or holding capacity increase on the basis of input parameters, which allow statements about temperatures and/or temperature changes of the brake system in the parts, regions and/or components for which the current holding capacity and/or holding capacity increase is to be determined. 
     The method according to the invention is intended to control a hydraulic brake system of a land vehicle, a brake system which has at least one wheel brake and a predetermined holding capacity for hydraulic fluid in predetermined parts, regions and/or components being assumed. In the method according to the invention, whether a current holding capacity of these parts, regions and/or components of the brake system has increased compared with the predetermined holding capacity is determined. If so, hydraulic fluid is fed to the at least one wheel brake in a controlled manner and an amount corresponding at least to the increase in capacity. 
     Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic representation of a brake system according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic representation of a preferred embodiment of a brake system. The brake system works by means of hydraulic fluid, which is partly stored in a container  11 . To generate braking pressure which results from the hydraulic fluid being put under pressure, a main cylinder  12 , which the driver can actuate through a pedal  13 , is used. As shown in the figures, optionally a brake servo  14 , which is arranged between the main cylinder  12  and the pedal  13 , can be provided, to amplify a force F which the driver introduces, preferably pneumatically or hydraulically. 
     Starting from the main cylinder  12 , a first and second brake circuit I, II are supplied, each brake circuit including two wheel brakes. Depending on which wheel brakes of the vehicle are included in which brake circuit, the result is a division between front and rear axles, in which one brake circuit can include wheel brakes of the front axle and the other brake circuit can include wheel brakes of the rear axle, or one brake circuit can include the wheel brake of one front wheel and the wheel brake of the diagonally opposite rear wheel and the other brake circuit can include the wheel brakes of the other front wheel and the other rear wheel. 
     Below, it is assumed that brake circuits I and II are of essentially identical structure. Therefore, only brake circuit I is shown in detail. The following explanations referring to brake circuit I apply correspondingly to brake circuit II. 
     Brake circuit I includes two wheel brakes  50  and  60 . To control braking pressure courses in the wheel brakes  50  and  60 , a first valve arrangement including valves  51  and  52  and a second valve arrangement including valves  61  and  62  are provided. 
     The valves  51 ,  52 ,  61  and  62  are shown here as 2/2-way valves which can be actuated by electromagnets. 
     Brake circuit I includes a valve device with valves  71 ,  72  and  73 . The valves  71  and  72  are shown here as 2/2-way valves which can be actuated by electromagnets. 
       FIG. 1  shows the valves  51 ,  61  and  71  each in an open operational state or through-flow position, whereas the valves  52 ,  62  and  72  are each shown in a closed operational state or blocked position. These operational states or positions are also called “initial position” below. 
     In the operational state of brake circuit I shown in  FIG. 1 , because of the open operational state of the valve  71 , there is a hydraulic connection between the main cylinder  12  and the wheel brakes  50  and  60 . This operational state is used for braking processes which can be controlled via actuations of the pedal  13 , and which can also be called normal braking processes. Actuations of the main cylinder  12  by means of the pedal  13 , optionally with the support of the brake servo  14 , generate braking pressures in brake circuit I and thus in the wheel brakes  50  and  60 . 
     The shown brake system is designed for so-called anti-lock regulation, in which, during a braking process, locking of the wheels should be prevented. For this purpose, braking pressures acting in the wheel brakes  50  and  60  are modulated individually. This is done by setting up pressure buildup, pressure holding and pressure reduction phases, which alternate in time sequence, and which are explained in more detail below. 
     The pressure buildup, pressure holding and pressure reduction phases are achieved by suitable control of the valves  51 ,  52  and  61 ,  62  which are associated with the wheel brakes  50  and  60  by means of an electronic control unit ECU. 
     The electronic control unit ECU can receive signals giving the operational states of a vehicle via a series of inputs (not shown). For instance, it is intended that signals from wheel rotational speed sensors, yaw velocity sensors, transverse acceleration sensors etc. should be fed to the electronic control unit ECU. 
     The electronic control unit ECU has outputs for control signals a, . . . , g to control the valves  51 ,  52 ,  61 ,  62 ,  71  and  72  and to control a motor  32 , which is provided to operate a pump  31 . 
     In the case of a driver-controlled normal braking process with anti-lock regulation, control is by the electronic control unit ECU, depending on measured values (e.g. speed, acceleration, wheel rotational speed, yaw velocity, transverse acceleration etc.) which indicate the operational states of the vehicle, and measured values which indicate a braking process which the driver wants (e.g. actuation of the pedal  13 , hydraulic pressure at the output of the main cylinder  12  etc.). The braking process which the driver wants can also be determined via the braking pressure P which is generated in the main cylinder  12 , to capture which a sensor  41  is provided. 
     During a normal braking process without anti-lock regulation, the valves  51 ,  52  and  61 ,  62  are each in their initial positions. If the electronic control unit ECU establishes, for instance, that the wheels associated with the wheel brakes  50  and  60  are tending to lock or locking, the electronic control unit ECU effects a pressure holding phase for each of the wheel brakes  50  and  60 . If the result of the pressure holding phases is not that the tendency to lock or locking ends, the electronic control unit ECU effects a pressure reduction phase for each of the wheel brakes  50  and  60 , until the tendency to lock or locking is ended. This is followed, under the control of the electronic control unit ECU, by pressure buildup phases for the wheel brakes  50  and  60 , in which the braking pressures acting in the wheel brakes  50  and  60  are built up according to the braking process which the driver wants. 
     During the pressure holding phases, under the control of the electronic control unit ECU, the valves  51  and  61  are each brought into a closed operational position or blocked position. The valves  52  and  62  remain in their initial positions which exist in the normal braking process. 
     Closing the valves  51  and  61  results in hydraulic disconnection of the wheel brakes  50  and  60 , so that the braking pressures acting in the wheel brakes  50  and  60  are kept constant. 
     During the pressure reduction phases, the valves  51  and  61  are held in their operational positions, and the valves  52  and  62  are controlled by the electronic control unit ECU so that they each take on an open operational state or through-flow position. Because of the open valves  52  and  62 , hydraulic fluid can flow out of the wheel brakes  50  and  60 , so that the braking pressures acting in the wheel brakes  50  and  60  are reduced. Hydraulic fluid which flows out can be stored temporarily in a low pressure reservoir  21 . 
     During a pressure buildup phase, the valves  51 ,  52  and  61  and  62  take their initial positions, i.e. the valves  51  and  61  are opened by the electronic control unit ECU, whereas the valves  52  and  62  are closed. To increase the braking pressures in the wheel brakes  50  and  60 , which were reduced in the pressure reduction phases, the electronic control unit ECU controls the motor  32  and thus the pump  31  so that via the valves  51  and  61 , the braking pressures acting in the wheel brakes  50  and  60  are raised to the level corresponding to the braking process which the driver wants. The pump  31  conveys hydraulic fluid which has flowed out in the pressure reduction phases back out of the low pressure reservoir  21  as required. 
     The pump  31 , which is implemented as, for instance, a radial piston pump, is blocking against its conveying direction, e.g. by means of a non-return valve  33  at the output of the pump  31  and a non-return valve  34  at the input of the pump  31 . 
     The rotational speed of the electric motor  32  can be adjusted and/or regulated via the control signal c of the electronic control unit ECU, so that the delivery rate of the pump  31  can be controlled. The electric motor  32  can simultaneously actuate a pump (not shown here) of the second brake circuit II. 
     Automatic braking processes usually take place independently of a force F which the driver introduces on the pedal  13 . These are, for instance, drive slip regulation, which prevents wheel spin of individual wheels in a startup process by targeted braking, driving dynamics regulation, which adjusts the vehicle behaviour in the limit range to the driver&#39;s wishes and the carriageway conditions by targeted braking of individual wheels, or adaptive speed regulation, which maintains a distance between one&#39;s own vehicle and a vehicle in front by, among other things, automatic braking. 
     For an automatic braking process, the valves  71  and  72  are controlled by the electronic control units ECU so that the valve  71  takes a closed operational state or blocked position, and the valve  72  takes an open operational state or through-flow position. The valves  51 ,  52 ,  61  and  62  remain in their initial positions. Because of the closed valve  71 , the open valves  51  and  61  and the closed valves  52  and  62 , the wheel brakes  50  and  60  are disconnected from the main cylinder  12  and brake circuit II to the extent that no braking pressures which are fed from outside, i.e. braking pressures which are caused by actuation of the pedal  13 , can be fed to them. Even the open valve  72  allows no feeding of external braking pressure, because the valves  52  and  62  are closed and the pump  31  also acts as a closed valve. However, the open valve  72  also enables the pump  31  to suck hydraulic fluid out of the container  11 , to generate braking pressures in the wheel brakes  50  and  60 , as described in detail below. 
     To generate braking forces which are wanted for automatic braking processes in the wheel brakes  50  and  60 , the electronic control unit ECU controls the motor  32  and pump  31  correspondingly. For modulation or fine adjustment of the braking pressures in the wheel brakes  50  and  60 , the electronic control unit ECU can control the valves  51 ,  52  and  61 ,  62  comparably to the anti-lock regulation described above. 
     To avoid damage to brake circuit I, the valve  73  can be provided in the form of a pressure limiting valve. The valve  73 , which is normally in a closed operational state, takes an open operational state which reduces braking pressure if the pressure at the output of the pump  31  is too high. 
     In operation of a brake system such as is described above, reduced or decreasing braking effects can occur. “Reduced or decreasing braking effect” should be understood here as meaning, in particular, that the brake system must be actuated more strongly or more and more strongly, to achieve a desired braking effect. Such worsened braking effects are also called “fading”, and in general occur when the brake system is very stressed. “The brake system is very stressed” should be understood here as meaning, in particular, that operation of the brake system results in heating components, particularly wheel brakes, which carry hydraulic fluid. The result of the heating can be an increased holding capacity or holding power of components which carry hydraulic fluid compared with the unheated state. The consequence is that more hydraulic fluid is required to generate a desired braking pressure. 
     In the case of automatic braking processes, this can be achieved by a pump of a brake circuit which has an increased holding capacity for hydraulic fluid being operated so that hydraulic fluid is fed until the desired braking pressure is reached. This has the disadvantage that, with equal pumping rate, the desired braking pressure is reached more slowly. Increasing the pumping rate to reduce the time until a desired braking pressure is generated, or to keep it equal compared with a state with normal holding capacity, is possible, if at all, only to a limited extent, depending on the design of the brake circuit and in particular of the pump. 
     In the case of braking processes in which braking pressure is at least partly built up by the driver, the quantity of hydraulic fluid which is required because of a capacity increase can be provided by the driver actuating the brake system more strongly, i.e. pressing the brake pedal further down. In this case, displacement reserves of brake system components which the driver has actuated directly and indirectly may not be enough to generate even high braking pressures, e.g. for full braking. Even if the driver, by corresponding stronger actuation of the brake system, can compensate for its increased holding capacity for hydraulic fluid, with equal actuation speed desired braking pressures are generated more slowly. To generate a desired braking pressure more quickly, the driver can actuate the brake system more quickly, i.e. press the brake pedal down more quickly, but this can result in unwanted, unpleasant and critical braking processes. However, these procedures are insufficient, at the latest, when no reserves are available to raise the braking pressure further. 
     To solve this problem, according to the invention, in general it is intended that, if in a brake system there is worsened braking effect because of increased holding capacity for hydraulic fluid, a quantity of hydraulic fluid corresponding at least to the capacity increase should be fed. 
     Below, to illustrate this procedure, preferred embodiments in which increased holding capacity for hydraulic fluid is established, on the one hand before a braking process and on the other hand during a braking process, are explained. 
     In one embodiment, whether the holding capacity of brake circuit I and/or brake circuit II has increased compared with the normal state, or it is to be assumed or expected that a holding capacity increase is present or can occur, is determined. To determine, estimate or predict a worsened braking effect of the brake position, various parameters can be used. For instance, it is intended that one or more of the following parameters should be used, in particular to determine, directly and indirectly, brake system temperatures which are relevant to a holding capacity increase: 
     temperatures of the wheel brakes  50 ,  60   
     number of actuations of the brake pedal  13  within a predefined period, optionally in association with braking pressures generated in the main cylinder  12   
     ratio of wheel rotational speeds and vehicle decelerations 
     pumping duration and/or pumping rate of the pump(s)  31   
     These parameters should be understood as examples only. Every parameter which allows statements about the holding capacity for hydraulic fluid can be used. 
     To determine, estimate or predict a holding capacity change, the control unit ECU receives, via one or more of its inputs, one or more input parameters, which allowed statements about the current holding capacity of the brake system, preferably about the current holding capacity of each brake circuit. 
     If the control unit ECU determines an operational state of the brake system which has resulted, is resulting or according to expectation can result in a worsened braking effect, the control unit ECU controls the brake system as explained below on the basis of brake circuit I. 
     In this embodiment, the brake system is controlled to compensate for a worsened braking effect by the control unit ECU, if the brake system is not being actuated. First, the control unit ECU controls the valve  71  and valve  72  so that the valve  71  takes a closed operational state or blocked position, and the valve  72  takes an open operational state or through-flow position. Also, the pump  31  is controlled to suck hydraulic fluid out of the hydraulic fluid reservoir or container  11  via the valve  72  and feed it to the wheel brakes  50  and  60 . The wheel brakes  50  and  60  are “prefilled” with a quantity of hydraulic fluid, to compensate for the increased holding capacity of brake circuit I, before a braking process takes place. In particular, it is intended here that a hydraulic fluid quantity corresponding to the capacity increase should be fed to the wheel brakes  50  and  60 . 
     If the quantity of hydraulic fluid to compensate for the increased holding capacity is fed, and if no braking process has (yet) been initiated, the control unit ECU can control the valve  72  so that it takes a closed operational position or blocking position. This prevents a flow of hydraulic fluid out of the wheel brakes  50  and  60 , and holds the hydraulic fluid which is fed for prefilling in the wheel brakes  50  and  60 . It is also provided that the control unit ECU controls the pump  31  so that it feeds no more hydraulic fluid, e.g. from the hydraulic fluid reservoir or low pressure reservoir  21 , to the wheel brakes  50  and  60 . 
     When the driver actuates the brake system, the control unit ECU controls the valve  71  so that it takes an open operational position or through-flow position. This makes it possible to build up braking pressure in the wheel brakes  50  and  60  by actuating the pedal  13  as described above. If the brake system is actuated by the driver, and if the valve  72  is closed to keep hydraulic fluid in the wheel brakes  50  and  60 , the control unit ECU keeps the valve  72  in this operational position. Otherwise, the control unit ECU brings the valve  72  out of the open operational position into the closed operational position or blocking position. It is also provided that the pump  31  is controlled so that it can be operated in the way which is intended for braking processes which the driver controls (e.g. anti-lock regulation). 
     What is achieved by the quantity of hydraulic fluid which is fed to the wheel brakes  50  and  60 , before the driver actuates the brake system, to compensate for the increased holding capacity, is that the quantity of hydraulic fluid which is fed to brake circuit I by actuating the pedal  13  is not used to compensate for the increased holding capacity, but essentially causes a braking pressure increase, which causes a braking effect, without loss. 
     If, before the wheel brakes  50  and  60  are prefilled, the brake system is operated for an automatic braking process, the valve  71  is kept in the closed operational position or blocking position. The valve  72  is also kept in its previous position, namely the open operational position or through-flow position, if the valve  72  was not closed, as described above, to keep hydraulic fluid in the wheel brakes  50  and  60 . Additionally, the control unit ECU controls the pump  31  so that a desired automatic braking process takes place. 
     Here the consequence of prefilling the wheel brakes  50  and  60  is that when a braking process is carried out, the pump  31  does not have to feed any hydraulic fluid to the wheel brakes  50  and  60  to compensate for the increased holding capacity, since this has already been done before the automatic braking process was initiated. 
     Preferably, the wheel brakes  50  and  60  are prefilled depending on operational or driving states of the land vehicle. For instance, it is provided that prefilling the wheel brakes  50  and  60 , if required, should be initiated, if the control unit ECU identifies an operational or driving state which causes a braking process to be expected. For instance, it is possible to feed parameters which indicate actuation-dependent positions and/or movements of an accelerator pedal to the control unit ECU, e.g. so that a change from the accelerator pedal to the brake pedal  13 , and thus a braking process to be expected, can be detected. Also other parameters, which indicate driving or operational states of the vehicle which may lead to a braking process are provided, e.g. critical driving states when driving round bends at high speed, distance from vehicles in front, and similar. 
     In another embodiment, in which no “predictive” prefilling of the wheel brakes  50  and  60  is provided, compensation for an increased holding capacity for hydraulic fluid takes place during a braking process. 
     During a braking process, to compensate for an increased holding capacity, in particular on the basis of brake system temperatures which are relevant to it, alternatively or additionally to the above-mentioned parameters, one or more of the following parameters can be used: 
     braking pressure generated in the main cylinder  12  during the braking process 
     brake temperature(s), in particular on a front axle 
     ratio of vehicle deceleration and braking pressure generated in the main cylinder  12   
     If the control unit ECU has detected a worsened braking effect because of increased holding capacity for hydraulic fluid, the control unit ECU controls the brake system as described above for an automatic braking process. In contrast to a normal automatic braking process, control here takes place so that a quantity of hydraulic fluid is fed to at least one wheel brake, which for this/these wheel brake(s) and a vehicle wheel which works with it/them results in an operational state which leads to anti-lock regulation. If the control unit ECU detects such an operational state which requires anti-lock regulation, the corresponding wheel brake is supplied with hydraulic fluid according to the anti-lock regulation described above. 
     Advantageously, for at least one wheel brake and/or wheel which is/are associated with a front axle of the vehicle, an operational state which requires anti-lock regulation is achieved. 
     The quantity of hydraulic fluid which is provided in this embodiment is fed independently of actuations of the brake system by means of the pedal  13 . Therefore, even in the case of a braking process which the driver controls, no braking pressure which the driver generates is used to compensate for the increased holding capacity. 
     Increasing the braking pressure by controlled feeding of hydraulic fluid out of the main cylinder  12 , so that anti-lock regulation is necessary in relation to at least one wheel brake, has the advantage that the additionally fed quantity of hydraulic fluid is actually enough to compensate for the increased holding capacity. In the case of a smaller quantity of fed hydraulic fluid, which does not lead to anti-lock regulation, it is possible that less hydraulic fluid is fed than is necessary to compensate for the increased holding capacity. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.