Abstract:
An electrohydraulic motor vehicle brake system is provided, having a hydraulic simulator circuit for generating a pedal feedback force, having a cylinder-piston unit and having an electromechanical actuator which acts on the cylinder-piston unit and which serves for generating a hydraulic pressure in at least one brake circuit, having a first fluid path with, arranged therein, a first valve device for the selective fluidic coupling of the cylinder-piston unit to the simulator circuit, and having a second fluid path with, arranged therein, a second valve device for the selective fluidic coupling of the simulator circuit to an unpressurized hydraulic fluid reservoir. Also specified are a ventilation method for a brake system of said type, a testing method for a further electrohydraulic motor vehicle brake system with only a first valve device, and a respective computer program product, with program code means, for carrying out one of the two methods when the computer program is executed on a processing unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the National Phase of International Application PCT/EP2014/072306 filed Oct. 17, 2014 which designated the U.S. and was published on Jun. 4, 2015 as International Publication Number WO 2015/078635 A1. PCT/EP2014/072306 claims priority to German Patent Application No. 10 2013 018 072.4, filed Nov. 28, 2013. Thus, the subject nonprovisional application claims priority to German Patent Application No. 10 2013 018 072.4, filed Nov. 28, 2013. The disclosures of these applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates generally to the field of brake systems. Specifically, an electrohydraulic motor vehicle brake system is described. Modern vehicle brake systems operate according to the “brake-by-wire” principle. This means that a hydraulic pressure is built up at the wheel brakes independently of foot force via a hydraulic pressure generator (e.g. a piston-cylinder device with an electromechanical actuator acting on the piston). Brake systems of this kind comprise, apart from the hydraulic pressure generator, a brake pedal interface with a sensor system for detection of an actuation of a brake pedal, a simulator circuit connected to the brake pedal interface for simulation of a realistic pedal response force and a plurality of valves for regulating the pressure. The valves are frequently accommodated in a block-shaped hydraulic control unit. Such brake systems are known, for example, from WO 2006/111393 A1, WO 2012/062393 A1 and WO 2012/152352 A1. 
         [0003]    Brake-by-wire brake systems have several advantages. For example, they are ideally suited for the installation of energy recovery systems. Furthermore, a build-up of brake pressure for an individual wheel can be controlled better by such brake systems and vehicle dynamics control programs (ABS, ASR, ESP programs) can be better integrated. 
         [0004]    On the other hand, brake-by-wire brake systems comprise a plurality of electrically actuatable hydraulic valves, which are arranged in a complicated network of fluid paths connected to one another. The fluid paths in turn are often only supplied via a central hydraulic fluid reservoir with hydraulic fluid. 
         [0005]    The servicing of such brake systems is complex on account of the plurality of valves and fluid guide paths. This is true, for example, of the ventilation of the brake system, as the wheel brakes, the simulator circuit and the hydraulic control unit have to be ventilated separately. The time outlay is correspondingly great in spite of the use of ventilation equipment, such as overpressure or vacuum equipment, for example. In the same way, detection of defective valves or of a leak in the brake system takes a lot of time, as normally all components of the simulator circuit and the brake circuits have to be checked individually in the context of fault location. 
       SUMMARY OF THE INVENTION 
       [0006]    The-invention provides a vehicle brake system in which the ventilation and checking of the brake system are made easier among other things. In addition, the invention provides a ventilation method and a test method for checking components of the brake system. 
         [0007]    According to a first aspect, an electrohydraulic vehicle brake system is provided, which comprises: a hydraulic simulator circuit for generation of a pedal response force; a first cylinder-piston device and an electromechanical actuator acting on the first cylinder-piston device for generation of a hydraulic pressure in at least one brake circuit; a first fluid path having a first valve unit arranged therein for a selective fluidic connection of the first cylinder-piston device to the simulator circuit; and a second fluid path having a second valve unit arranged therein for a selective fluidic connection of the simulator circuit to an unpressurised hydraulic fluid reservoir. 
         [0008]    The brake system can comprise at least one hydraulic brake circuit connectable fluidically to the first cylinder-piston device. Furthermore, the first end of the first fluid path can lead into the simulator circuit and the second end can lead into the at least one brake circuit. In this case the simulator circuit is connected fluidically via the first fluid path and the at least one brake circuit to the first cylinder-piston device. According to an alternative variant, the first fluid path can also lead directly (i.e. independently of the at least one brake circuit) into the first cylinder-piston device. In this case the simulator circuit can be connected fluidically via the first fluid path directly to the electromechanically actuatable first cylinder-piston device. 
         [0009]    The first cylinder-piston device and the electromechanical actuator acting on the first cylinder-piston device can form an autonomous hydraulic pressure generator, which can build up a hydraulic pressure in the brake system independently of foot force. According to one implementation, the first cylinder-piston device can be formed as the main brake cylinder of the brake system, which can additionally still be actuated by foot force if the actuator fails. According to an alternative configuration, the first cylinder-piston device can be provided in addition to a main brake cylinder of the brake system for the generation of hydraulic pressure independently of foot force in the brake system. 
         [0010]    According to a further configuration, the first cylinder-piston device can be connected fluidically to a second cylinder-piston device of the brake system, in order to build up a hydraulic pressure at the wheel brakes of the brake system. In this case the first cylinder-piston device can be provided to supply a hydraulic pressure for a hydraulic actuation of a piston of the second cylinder-piston device. By the hydraulic actuation of the piston of the second cylinder-piston device, a hydraulic pressure is then built up at the wheel brakes. The second cylinder-piston device can be formed as the main brake cylinder of the brake system, for example. 
         [0011]    The first valve unit can be arranged parallel to the second valve unit. Correspondingly the first fluid path and the second fluid path can also be formed differently from one another. Specifically the first end of the first fluid path can lead into the simulator circuit and its second end can lead into the at least one brake circuit or directly into the first cylinder-piston device. Correspondingly the first end of the second fluid path can lead into the unpressurised hydraulic fluid reservoir and its second end into the simulator circuit. The first hydraulic path and the second hydraulic path can thus be connected fluidically via the simulator circuit. The first fluid path, the simulator circuit and the second fluid path can together form a fluid circuit of the brake system, which facilitates a fluidic connection of the first cylinder-piston device to the simulator circuit and/or to the unpressurised hydraulic fluid reservoir depending on the switching of the first valve unit and the second valve unit. With suitable activation of the first valve unit, the second valve unit and the actuator, the fluid circuit can be used to ventilate the brake system. However, the fluid circuit can also be used for testing brake system components, as described in greater detail below. 
         [0012]    The simulator circuit can comprise a hydraulic pressure accumulator, which is connected fluidically to a brake pedal interface. The hydraulic pressure accumulator can be connected fluidically in this case to the pedal interface via a fluid path. The hydraulic pressure accumulator can be realised as a pressure vessel in the form of a piston-cylinder arrangement acted upon by spring force, wherein the piston taken up displaceably in the cylinder is pretensioned by a spring. 
         [0013]    The first valve unit can be formed to be in a closed state in the unactuated state. In other words, the first valve unit can block the first fluid path in the unactuated state. The simulator circuit can thus be disconnected fluidically from the first cylinder-piston device. The first valve unit can be electrically actuatable. The first valve unit can be switched by electrical activation from a closed to an open state. The valve unit can comprise one or more electrically actuatable valves. 
         [0014]    The second valve unit can be formed to be in an open state in the unactuated state. In the unactuated state the second valve unit can assume an open valve position, whereby the simulator circuit is connected fluidically to the hydraulic fluid reservoir. The second valve unit can comprise at least one electrically actuatable valve. The second valve unit can be switched by electrical activation of the at least one valve of the second valve unit to a closed state. The simulator circuit can be disconnected fluidically from the hydraulic fluid reservoir in this way. 
         [0015]    The electrohydraulic brake system can also comprise an electric control apparatus or control apparatus system for the electrical activation of the first valve unit and for electrical activation of the electromechanical actuator. The control apparatus or control apparatus system can be formed to execute the following actuation steps (by electrical activation): switching of the first valve unit from a closed state to an open state, to connect the simulator circuit fluidically to the at least one brake circuit; and operation of the electromechanical actuator in order to displace hydraulic fluid from the cylinder-piston device via the first fluid path into the simulator circuit. 
         [0016]    The control apparatus or control apparatus system can also be designed for the electrical activation of the second valve unit. The control apparatus can be designed to switch the second valve unit by electrical activation from an open state to a closed state before hydraulic fluid is displaced into the simulator circuit. The hydraulic fluid displaced via the first fluid path and the first valve unit into the simulator circuit can thus not flow into the hydraulic fluid reservoir via the second fluid path and the valve unit arranged in it. On the contrary, the fluid volume displaced by the actuator is stored in the hydraulic pressure accumulator of the simulator circuit. 
         [0017]    The control apparatus or control apparatus system can also be formed to switch the second valve unit back to an open state following displacement of a predetermined volume of hydraulic fluid into the simulator circuit by electrical activation. In this way the fluid volume stored intermediately in the hydraulic pressure accumulator of the simulator circuit can flow via the second fluid path and the second valve unit arranged therein into the hydraulic fluid reservoir. The speed at which the second valve unit allows the volume of hydraulic fluid stored in the hydraulic pressure accumulator to flow via the second fluid path can be used to determine the flow properties of the second valve unit. 
         [0018]    Alternatively to the aforementioned closure of the second valve unit, the second valve unit can remain switched in an open state during the operation of the electromechanical actuator. If the second valve unit should be in a closed state initially upon operation of the electromechanical actuator, the control apparatus can switch the second valve unit to an open state. Hydraulic fluid conveyed from the first cylinder-piston device can thus pass initially via the first fluid path into the simulator circuit. From there, the hydraulic fluid conveyed can be conducted away via the second fluid path into the hydraulic fluid reservoir. The simulator circuit and the pedal interface connected fluidically to the simulator circuit can thus be ventilated in this way independently of the driver. The conveying of hydraulic fluid via the first fluid path into the simulator circuit and from there via the second fluid path back into the hydraulic fluid reservoir can also be used for testing other valves connected downstream of the second valve unit. 
         [0019]    The electrohydraulic brake system can further comprise at least a first detection device. The at least one first detection device can be formed to detect a hydraulic pressure built up in the brake circuit and/or the simulator circuit during the operation of the electromechanical actuator. The detection device can comprise at least one pressure sensor, which detects the pressure in the brake circuit and/or in the simulator circuit during the conveying of the hydraulic fluid. In addition or alternatively to this, the first detection device can comprise a sensor that detects a measured variable indicating a pressure (indirect pressure measurement). 
         [0020]    The electrohydraulic brake system can further comprise at least one second detection device, which is formed to detect a volume of hydraulic fluid displaced from the first cylinder-piston device due to an operation of the electromechanical actuator. The second detection device can detect an actuator parameter in this case, which indicates an actuator operation and thus an actuation of the piston on which the actuator acts. The volume of fluid conveyed can be determined in a known manner from the detected piston actuation and a radius of the cylinder chamber of the first cylinder-piston device. 
         [0021]    The electrohydraulic brake system can further comprise a comparison device, which is formed to compare the detected hydraulic pressure and the detected volume of fluid conveyed with a set pressure-volume characteristic. The set pressure-volume characteristic can reproduce the rise in pressure to be expected as a function of the displaced volume of fluid of a ventilated and fully functional simulator circuit. By comparison of the measured pressure-volume characteristic with the stored pressure-volume characteristic, the force response of the hydraulic pressure accumulator in the simulator circuit can be tested. In particular, the level of ventilation of the simulator circuit can be determined from the comparison of the detected pressure-volume characteristic with a stored pressure-volume characteristic. The set pressure-volume characteristic can be stored in a memory of the comparison device. In particular, the comparison device can be integrated in the control apparatus in the form of a software module. 
         [0022]    The electrohydraulic brake system can further comprise at least a third valve unit arranged in the at least one brake circuit for the selective fluidic connection of a wheel brake to the first cylinder-piston device or the second cylinder-piston device. The at least one third valve unit can be electrically actuated via the control apparatus, in order to connect or disconnect fluidically each wheel brake of the brake system as required to or from the first cylinder-piston device or the second cylinder-piston device. For example, the control apparatus or control apparatus system can activate the third valve unit in brake operation of the brake system in such a way (e.g. activation according to a time multiplex method) that a quite definite hydraulic pressure can be set at the wheel brakes of the brake system. 
         [0023]    However, the control apparatus or control apparatus system can also activate the third valve unit as part of a test method for the brake system or as part of an automatic brake system ventilation method, in order, if required, either to disconnect fluidically or to connect fluidically the wheel brakes from/to the first cylinder-piston device or, in a serial arrangement of the first and second cylinder-piston device, from/to the second cylinder-piston device. For example, the control apparatus or the control apparatus system can be formed to switch the at least one third valve unit from an open state to a closed state before operation of the electromechanical actuator as part of a brake system test method or a ventilation method. It can be ensured in this way that the hydraulic fluid conveyed from the hydraulic pressure generator does not reach the wheel brakes during a test function of the brake system. The hydraulic fluid conveyed from the first cylinder-piston device can thus be conveyed directly into the simulator circuit when the third valve unit is closed and the first valve unit is open. Alternatively to this, the third valve unit can also remain in an open state as part of a ventilation method. If the third valve unit was previously in a closed state, it can be switched to an open state by the control apparatus. 
         [0024]    According to a second aspect, a method for checking a functionality of an electrohydraulic vehicle brake system is provided. The electrohydraulic vehicle brake system comprises a hydraulic simulator circuit for generation of a pedal response force, a first cylinder-piston device and an electromechanical actuator interacting with the first cylinder-piston device for the generation of a hydraulic pressure in at least one hydraulic brake circuit and a first fluid path having a first valve unit arranged therein for the selective fluidic connection of the first cylinder-piston device with the hydraulic simulator circuit. The method comprises the steps of switching the first valve unit from a closed state to an open state, in order to connect the hydraulic simulator circuit fluidically to the first cylinder-piston device, operation of the electromechanical actuator, in order to displace hydraulic fluid from the first cylinder-piston device via the first hydraulic fluid path into the simulator circuit, the detection of a hydraulic pressure prevailing on account of the displaced hydraulic fluid, and checking of the functionality of the electrohydraulic vehicle brake system on the basis of the hydraulic pressure detected. 
         [0025]    The hydraulic pressure generated can be detected in this case in the first cylinder-piston device and/or in the brake circuit and/or in the simulator circuit. The hydraulic pressure can be detected during the operation of the actuator, in order to detect a pressure build-up in the brake circuit or simulator circuit, for example. In addition, the pressure can also be detected if the operation of the actuator is (temporarily) stopped and the piston is located in a position moved forward in the cylinder. The pressure generated by actuation of the piston as well as possible temporal pressure changes (e.g. a temporal pressure drop due to a leak in the simulator circuit) can be detected in this way. The pressure can be detected continuously or at predetermined intervals (e.g. at intervals of 0.1 seconds). 
         [0026]    The method can also comprise the step of detection of the volume of hydraulic fluid displaced during the operation of the actuator. The detection of the volume of hydraulic fluid conveyed can, like the pressure detection, take place continuously or at set intervals (e.g. at intervals of 0.1 seconds). Furthermore, the detection of the volume of hydraulic fluid can take place substantially synchronously with the pressure detection. It is possible in this way to obtain a sequence of substantially contemporaneously recorded pressure values and fluid volume values during a piston actuation. 
         [0027]    The step of checking can also comprise a comparison of the hydraulic pressure detected and the volume of hydraulic fluid detected with a set pressure-volume characteristic. The set pressure-volume characteristic An unambiguous pressure-volume relationship can be deduced from the substantially contemporaneously detected pressure values and fluid volume values. This detected functional relationship between pressure and volume conveyed can be compared with the stored characteristic. It can be established in this way whether deviations occur between the measured pressure-volume characteristic and the set pressure-volume characteristic. Conclusions can be drawn from the detected deviations between the measured pressure-volume characteristic and the set pressure-volume characteristic about the degree of ventilation or the wear of the simulator circuit. 
         [0028]    The vehicle brake system can further comprise a second hydraulic fluid path having a second valve unit arranged therein for the selective fluidic connection of the simulator circuit to an unpressurised hydraulic fluid reservoir. The second valve unit can be an electrically actuatable valve unit. When unactuated (i.e. de-energised) it can assume an open valve position and in the energised state a closed valve position. In this case the method can additionally comprise the step of switching the second valve unit from an open state to the closed state before hydraulic fluid is displaced into the simulator circuit, in order to dam up the displaced hydraulic fluid in the simulator circuit. The hydraulic fluid conveyed from the cylinder-piston device is stored initially in a hydraulic pressure accumulator of the simulator circuit in this way. The pressure generated in this case can be detected as a function of the displaced volume of hydraulic fluid and used for checking the simulator circuit, as already described above. 
         [0029]    The method can also comprise the step of opening of the second valve unit after a volume of hydraulic fluid has been displaced into the simulator circuit. The dammed up hydraulic fluid in the simulator circuit can then drain via the second fluid path and the second valve unit into the hydraulic fluid reservoir. The (temporal) decrease in pressure in the simulator circuit and in the at least one brake circuit (or in the cylinder of the pressure generator) can be detected. The flow properties of the second valve device (and of the second fluid path) can be tested on the basis of the detected temporal decrease in the hydraulic pressure. 
         [0030]    The vehicle brake system can further comprise a second hydraulic fluid path having a second valve unit arranged therein for the selective fluidic connection of the simulator circuit to a hydraulic fluid reservoir, wherein in the case of the second valve unit a pressure-controlled valve unit is connected downstream. In order to determine a switching pressure of the pressure-controlled valve unit, for example, the method according to one test variant can provide that the second valve unit is not switched on operation of the actuator and thus remains in an open state. The hydraulic fluid conveyed from the cylinder can thus reach the valve inlet of the pressure-controlled valve unit via the first fluid path, via the brake circuit and via the second fluid path. The hydraulic fluid can then be dammed up in the test circuit until the pressure-controlled valve unit switches to an open state on account of the pressure that has built up. Only then can the hydraulic fluid from the test circuit flow into the hydraulic fluid reservoir and the pressure does not rise any further. The switching pressure of the pressure-controlled valve unit can be determined from the measured pressure rise and the sudden interruption in the pressure rise. 
         [0031]    Furthermore, the brake system can comprise at least one brake circuit that can be connected fluidically to the first cylinder-piston device or to the second cylinder-piston device and at least one third valve unit arranged in the at least one brake circuit for the selective fluidic connection of a wheel brake to the first or second cylinder-piston device. In this case the method can additionally comprise the step of switching the third valve unit from an open state to a closed state before the actuator is operated. In this way no hydraulic fluid can get to the wheel brakes of the brake system during the test method. It can thereby be prevented that the wheel brakes are operated during the test method and thus distort the pressure detection. 
         [0032]    According to another aspect, a method is provided for ventilation of a hydraulic simulator circuit of an electrohydraulic vehicle brake system. The brake system comprises a hydraulic simulator circuit for generation of a pedal response force, a first cylinder-piston device and an electromechanical actuator acting on the first cylinder-piston device for generation of hydraulic pressure in at least one brake circuit, a first fluid path having a first valve unit arranged therein for the selective fluidic connection of the at least one brake circuit to the simulator circuit and a second fluid path having a second valve unit arranged therein for the selective fluidic connection of the hydraulic simulator circuit to an unpressurised hydraulic fluid reservoir. The method comprises the following steps: opening of the first valve unit, in order to connect the first cylinder-piston device fluidically to the simulator circuit; opening of the second valve unit, in order to connect the simulator circuit fluidically to the hydraulic fluid reservoir, if the second valve unit was in a closed state; and operation of the electromechanical actuator in order to displace hydraulic fluid via the simulator circuit into the hydraulic fluid reservoir. 
         [0033]    The vehicle brake system can further comprise at least one brake circuit connectable fluidically to the first cylinder-piston device or to the second cylinder-piston device and at least one third valve unit arranged in the at least one brake circuit for the selective fluidic connection of a wheel brake to the first or second cylinder-piston device. In this case, the method according to one variant comprises switching of the third valve unit from an open state to a closed state before the actuator is operated. 
         [0034]    According to an alternative variant, the third valve unit can remain switched to an open state and the first valve unit to a closed state on operation of the electromechanical actuator. Hydraulic fluid is then conveyed initially to the wheel brakes of the at least one brake circuit. The first valve unit can then be opened in order to let the hydraulic fluid stored in the wheel brakes and/or brake circuits drain via the simulator circuit into the fluid reservoir. 
         [0035]    The aforementioned test methods and ventilation methods can be executed during a brake-operation-free phase. Furthermore, all methods can be executed automatically and thus completely independently of the driver. 
         [0036]    According to another aspect, a computer program product is provided, with program code means for executing the methods presented here when the computer program product runs on a processor. The computer program product can be stored on a computer-readable data carrier. 
         [0037]    Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1A  is an embodiment of an electrohydraulic vehicle brake system; 
           [0039]      FIG. 1B  is another embodiment of an electrohydraulic vehicle brake system; 
           [0040]      FIG. 2  is another embodiment of an electrohydraulic vehicle brake system; 
           [0041]      FIG. 3A / 3 B is an embodiment of a pressure-controlled valve unit of the electrohydraulic vehicle brake system; 
           [0042]      FIG. 4  is a flow chart for representing a test method for the vehicle brake systems according to  FIGS. 1 and 2 ; 
           [0043]      FIG. 5  is the vehicle brake system according to  FIG. 2  for illustrating the test method according to  FIG. 4 ; 
           [0044]      FIG. 6  is a flow chart for illustrating another test method for the vehicle brake systems according to  FIGS. 1 and 2 ; 
           [0045]      FIG. 7  is a flow chart for illustrating another test method for the vehicle brake systems according to  FIG. 2 ; 
           [0046]      FIG. 8  is the vehicle brake system according to  FIG. 2  for illustrating the test method according to  FIG. 7 ; 
           [0047]      FIG. 9  is a flow chart for illustrating a ventilation method for the vehicle brake systems according to  FIGS. 1 and 2 ; 
           [0048]      FIG. 10  is the electrohydraulic vehicle brake system according to  FIG. 2  for illustrating the ventilation method according to  FIG. 8 ; 
           [0049]      FIG. 11  is a flow chart for illustrating another ventilation method for the vehicle brake systems according to  FIGS. 1 and 2 ; and 
           [0050]      FIG. 12A / 12 B is the electrohydraulic vehicle brake system according to  FIG. 2  for illustrating the ventilation method according to  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0051]    Referring now to the drawings,  FIG. 1A  shows an electrohydraulic vehicle brake system  1000  with two brake circuits  10 ,  20 . The vehicle brake system  1000  comprises a hydraulic pressure generator assembly  100 , a first fluid path  340 , a valve unit  330  arranged in the first fluid path  340 , a second fluid path  140 ,  140   a ,  140   b , a second valve unit  130  arranged in the second fluid path  140 ,  140   a ,  140   b , a simulator circuit  145 , an electronic control apparatus or an electronic control unit  200  (termed ECU below), a hydraulic control unit  300  (termed HCU below) and wheel brakes  401 - 404  respectively connected fluidically to a first brake circuit  10  and a second brake circuit  20 . The brake system  1000  also comprises two return lines  30 ,  40 , which are each fluidically connected at a first end to a corresponding brake circuit  10 ,  20  and lead at a second end into an unpressurised hydraulic fluid reservoir  170 . 
         [0052]    Optionally to this, the brake system can comprise a generator unit  500  for energy recovery. The generator unit  500  is designed to convert kinetic energy back into electrical energy in brake operation. The generator unit  500  is connected for this to at least one wheel  400 , in order to convert the rotational movement into electrical energy. The generator unit  500  can also be connected to an energy store (e.g. a battery, not shown in  FIG. 1A ), which stores the electrical energy obtained. 
         [0053]    The HCU  300  comprises a plurality of valve groups for hydraulic pressure regulation in the two brake circuits  10 ,  20 . Deviating from the separate formation shown in  FIG. 1  of HCU  300 , the first valve unit  330  and the second valve unit  130 , the first valve unit  330  and the second valve unit  130  can also be integrated into the HCU  300 . Examples for the realisation of the valves and valve groups of the HCU  300  are described in greater detail further below in connection with  FIG. 2 . 
         [0054]    The ECU  200  is designed at least to activate electrically actuatable valves of the HCU  300 . It is also designed to activate the assembly  100 . For this purpose the ECU  200  comprises control functions for the HCU  300  and the assembly  100 . Alternatively to this, it is also conceivable that the activation functions for the assembly  100  and HCU  300  are each organised in separate electronic control sub-units and these sub-units accordingly interact in brake operation. 
         [0055]    The structure and mode of operation of the hydraulic pressure generator assembly  100  are explained in greater detail below. First an implementation option for the assembly  100  is described in greater detail with reference to  FIG. 1A . The hydraulic pressure generator assembly  100  comprises a main brake cylinder  110 , a pedal interface  115  with a hydraulic cylinder  120 , a power transmission device  150  and an electromechanical actuator  160 . According to one variant, the unpressurised hydraulic fluid reservoir  170  for storing hydraulic fluid can be integrated into the assembly  100 . 
         [0056]    The main brake cylinder  110  is formed in the embodiment as a tandem main brake cylinder  110 . A piston arrangement  112 ,  114  taken up displaceably in the main brake cylinder  110  comprises a first piston  112  (termed primary piston below) and a second piston  114  (termed secondary or floating piston below). Here a first hydraulic chamber  116  is defined by the end face (left end face) of the first piston  112  pointing in the travel direction and by the end face of the second piston  114  pointing opposite to the travel direction. Furthermore, a second hydraulic chamber  118  is defined by the end face of the secondary piston  114  pointing in the travel direction and the cylinder base. Both hydraulic chambers  116 ,  118  are respectively connected fluidically to a brake circuit  10 ,  20  of the brake system  1000  (i.e. to the first brake circuit  10  or to the second brake circuit  20 ). The hydraulic chambers  116 ,  118  also have a fluidic connection in a known manner to the unpressurised hydraulic fluid reservoir  170 . 
         [0057]    By actuation of the primary piston  112  and the secondary piston  114 , hydraulic fluid can be conveyed from the two chambers  116 ,  118  into the respective brake circuits  10 ,  20  and a hydraulic pressure thus generated at the wheel brakes  401 - 404 . The actuation can be carried out by the driver via the brake pedal  126  or via the electromechanical actuator  160 , as described in greater detail further below. If the actuation of the piston arrangement  112 ,  114  is carried out exclusively via the actuator  160 , then the assembly  100  is in normal operation. A braking process then takes place according to the brake-by-wire principle. If the actuation of the piston arrangement  112 ,  114  is by foot force via the brake pedal  126 , on the other hand, the assembly  100  is in emergency operation (push-through operation). 
         [0058]    The hydraulic pressure built up at the wheel brakes  401 - 404  in the context of normal operation or push-through operation can be relieved again via the return lines (fluid paths  30 ,  40 ). These lead directly into the unpressurised hydraulic fluid reservoir  170  and enable hydraulic fluid from the wheel brakes  401 - 404  to flow back via the fluid paths  30 ,  40  into the unpressurised hydraulic fluid reservoir  170 . 
         [0059]    It is understood that the present disclosure does not depend on the design details of the main brake cylinder  110 . Instead of the tandem main brake cylinder  110  described here, the main brake cylinder can be formed in a “twin” arrangement, in which the hydraulic chambers  116 ,  118  assigned to the two brake circuits  10 ,  20  are arranged parallel to one another. Furthermore, the main brake cylinder  110  can have more than two hydraulic chambers  116 ,  118  and the brake system  1000  can comprise more than two brake circuits  10 ,  20 . 
         [0060]    The pedal interface  115  comprises the hydraulic cylinder  120 , a piston  122  taken up displaceably in the hydraulic cylinder  120 , a first plunger  123  and a second plunger  125 . The first plunger  123  is attached to an end face of the piston  122  facing the travel direction. The first plunger  123  is arranged in this case coaxially with the piston  122 . The first plunger  123  is provided to transmit an actuation of the brake pedal  126  to the power transmission device  150 . 
         [0061]    The second plunger  125  is attached at a first end to an end face of the piston  122  facing a brake pedal  126  (thus the end face facing away from the travel direction). The second plunger  125  is likewise arranged coaxially with the piston  122 . The second end of the second plunger  125  is connected mechanically to the brake pedal  126 . In this way a pedal actuation (i.e. pressing the brake pedal  126  down) can be transmitted to the piston  122 , which is then displaced in the direction of travel (to the left in  FIG. 1A ). The pedal actuation (movement and the force applied for the movement) can be transmitted to the power transmission device  150  by the first plunger  123 . 
         [0062]    Together with the hydraulic cylinder  120 , the end face of the piston  122  pointing in the travel direction also defines a hydraulic chamber  124 , which is filled with hydraulic fluid. On actuation of the brake pedal  126 , the piston  122  is displaced in the travel direction, due to which hydraulic fluid is displaced from the chamber  124 . The displaced hydraulic fluid can be displaced in this case into the simulator circuit  145  or (at least partially) into the at least one brake circuit  10 ,  20  and the main brake cylinder  110 , as described in greater detail below. 
         [0063]    The power transmission device  150  is arranged between the main brake cylinder  110  and the hydraulic cylinder  120 . It comprises at least one piston rod  151 . The piston rod  151  is arranged coaxially with the piston arrangement  112 ,  114  of the main brake cylinder  110  and with the piston  122  of the hydraulic cylinder  120 . It extends through a spindle  162  (formed as a hollow body) of the electromechanical actuator  160  and can be brought to abut at a first end (left end in  FIG. 1A ) on at least the primary piston  112  of the piston arrangement  112 ,  114 . The second end of the piston rod  151  can also be brought to abut on the first plunger  123  of the pedal interface  115 . 
         [0064]    As also shown in  FIG. 1A , the second end of the piston rod  151  is separated spatially from the first plunger  123  by a void (or gap)  152 . This void  152  is maintained in normal operation of the assembly  100  due to a displacement carried out by the actuator  160  of the primary piston  112  and the piston rod  151  in the travel direction, even on depression of the brake pedal  126 . The first plunger  123  thus does not come into contact with the piston rod  151  during normal operation of the assembly  100 , and the actuation force acting on the brake pedal  126  cannot be transmitted to the piston rod  151 . 
         [0065]    In emergency operation of the assembly  100  the actuator  160  remains unactuated. The piston rod  151  is thus not displaced by the actuator  160 . On depression of the brake pedal  126  the (small) void  152  between the first plunger  123  and the second end of the piston rod  151  can quickly be overcome. The first plunger  123  comes into contact with the piston rod  151 . The piston rod  151  then transmits the displacement of the piston  122  in the direction of the piston rod  151  that is caused when the brake pedal  126  is pressed directly to the primary piston  112  of the main brake cylinder  110  (push-through principle). The primary piston  112  transmits the movement in turn to the secondary piston  114 . By actuation of primary piston  112  and secondary piston  114  a hydraulic pressure can then be built up at the wheel brakes  401 - 404 . Overall the mechanical power transmission device  150  described here facilitates a direct mechanical connection of the primary piston  112  to the piston  122  of the pedal interface  115  and the brake pedal  126  to build up hydraulic pressure during emergency operation (i.e. if no hydraulic pressure can be built up via the actuator  160 ). 
         [0066]    According to the variant shown in  FIG. 1A , the electromechanical actuator  160  is likewise arranged between the main brake cylinder  110  and the hydraulic cylinder  120 . Main brake cylinder  110 , electromechanical actuator  160  and hydraulic cylinder  120  are arranged in series and coaxially with a cylinder axis predetermined by the main brake cylinder  110 . The electromechanical actuator  160  is provided to act on the piston arrangement  112 ,  114  of the main brake cylinder  110  in order to be able to build up a hydraulic pressure in the brake system  1000  independently of foot force. In normal operation of the assembly  100 , the hydraulic pressure is built up exclusively via the electromechanical actuator  160 . 
         [0067]    The electromechanical actuator  160  comprises an electric motor  161  as well as a gear unit  162 ,  163  connected to the electric motor  161  to transmit the motor movement to the piston arrangement  112 ,  114  of the main brake cylinder  110 . In the practical example, the gear unit is an arrangement of a rotatably supported nut  163  and a spindle  162  that is in engagement with the nut  163  (e.g. via rolling elements such as balls) and is movable in an axial direction. In other executions, toothed racks or other gear types can be used. 
         [0068]    The electric motor  161  in the present embodiment has a cylindrical shape and extends concentrically to the transmission device  150 . More precisely, the electric motor  161  is arranged radially externally with reference to the piston rod  151  of the transmission device  150 . A rotor (not shown) of the electric motor  161  is connected non-rotatably to the gear nut  163  in order to set this in rotation. A rotary movement of the nut  163  is transmitted to the spindle  162  in such a way that an axial displacement of the spindle  162  results. The left end face of the spindle  162  in  FIG. 1A  can come into contact (if applicable via an intermediate element) with the right end face of the primary piston  112  in  FIG. 1A  and as a consequence of this displace the primary piston  112  (together with the secondary piston  114 ) to the left and thus convey hydraulic fluid into the brake circuits  10 ,  20 . Conversely, the piston arrangement  112 ,  114  can be brought, when the spindle  162  travels back (thus moves to the right, opposite to the travel direction), into an unactuated state (starting position) via the hydraulic pressure prevailing in the chambers  116 ,  118  and/or the reset springs arranged in the chambers  116 ,  118 . 
         [0069]    The electromechanical actuator  160  is thus suitable for displacing the primary piston  112  of the main brake cylinder  110  autonomously (i.e. independently of foot force) in order to build up a hydraulic pressure at the wheel brakes  401 - 404  according to the brake-by-wire principle. The extent of the pressure build-up can be determined by at least one activation value of the ECU  200 , which contains information on how strongly the actuator  160  should be actuated (and the spindle  162  displaced in the travel direction). The activation value can be determined on the basis of a sensor system (e.g. by means of path and/or force sensors) connected to the brake pedal  126  or the pedal interface  115 . 
         [0070]    In the variant of the assembly  100  shown in  FIG. 1A , a hydraulic pressure can be built up in the at least one brake circuit  10 ,  20  via the main brake cylinder  110  and the piston arrangement  112 ,  144  taken up displaceably in it, in that the piston arrangement  112 ,  114  is actuated either via an actuation force applied by the electromechanical actuator  160  or by an actuation force applied by the driver to the brake pedal  126  and actuation force transmitted via the piston-plunger arrangement  122 ,  123 ,  125  of the pedal interface  115  and via the power transmission device  150 . In the former operation mode of the piston arrangement  112 ,  114  the assembly  100  is in normal operation, in the latter operation mode in push-through operation. 
         [0071]    According to an alternative variant, the electromechanical actuator  160  of the assembly  100  can act on a different cylinder-piston device from the main brake cylinder  110  to build up a hydraulic pressure at the wheel brakes  401 - 404  independently of foot force. In this case the cylinder-piston device is arranged in addition and by way of example parallel to the main brake cylinder  110 , as is known from the printed publication WO 2011/141158 A2, the teaching of which is hereby included. Alternatively to this arrangement, the use of an electrohydraulic actuator (e.g. a hydraulic pump) is also conceivable, in order—following the brake-by-wire principle—to convey hydraulic fluid into the first and second brake circuit  10 ,  20 . The present disclosure does not depend on the form and arrangement of the actuator  160  used to realise a brake-by-wire operation. 
         [0072]    The simulator circuit  145  already mentioned comprises a hydraulic pressure accumulator  144 , which is connected fluidically to the hydraulic cylinder  120  via a fluid path  141  (and a throttle valve or throttle check valve arranged therein). The hydraulic pressure accumulator  144  is realised as a piston-cylinder arrangement, wherein the piston taken up displaceably in the cylinder is pretensioned by a spring. 
         [0073]    On actuation of the brake pedal  126  in normal operation, the hydraulic fluid conveyed from the hydraulic cylinder  120  is conducted via the fluid path  141  into the hydraulic pressure accumulator  144 . The fluid flowing into the hydraulic pressure accumulator  144  displaces the piston pretensioned by the spring. The force to be applied for the displacement of the piston retroacts as pedal reset force on the brake pedal  126 . In other words, the hydraulic pressure accumulator  144  generates a counterpressure, which retroacts on the piston  122  and on the brake pedal  126 . In this way a counterforce acting on the brake pedal  126  is produced, which in brake-by-wire operation prevents the brake pedal  126  from being slack and gives the driver a realistic brake pedal sensation. 
         [0074]    As shown schematically in  FIG. 1A , the brake system  1000  comprises the first fluid path  340  with the first valve unit  330  arranged in it. The fluid path  340  and the valve unit  330  arranged therein are formed to connect the main brake cylinder  110  selectively fluidically to the simulator circuit  145 . To do this, the first end of the first fluid path  340  can lead into the simulator circuit  145  (more precisely into the fluid path  141 ) and its second end can lead into the at least one brake circuit  10 ,  20  (for example into the first brake circuit  10 , as shown in  FIG. 1A ). According to an alternative execution, the second end of the fluid path  340  can also lead directly into the main brake cylinder  110 . It is only essential that a fluidic connection is realisable between the hydraulic pressure generator of the assembly  100  and the simulator circuit  145  via the first fluid path  340 . In this connection it should be noted that, according to the variant of the electrohydraulic brake system  1000  shown in  FIG. 1A , the hydraulic pressure generator is realised by the main brake cylinder  110 , the piston arrangement  112 ,  114  arranged in the main brake cylinder  110  and the electromechanical actuator  160  acting on the piston arrangement  112 ,  114 . It is understood that in the brake system described in the printed publication WO 2011/141158 A2, the hydraulic pressure generator is realised in the form of a cylinder-piston arrangement different from the main brake cylinder and an electromechanical actuator acting on this cylinder-piston arrangement. 
         [0075]    The first valve unit  330  is formed to assume an open or a closed state depending on activation, in order to connect or disconnect the simulator circuit  145  fluidically from the first brake circuit  10  and/or from the main brake cylinder  110 . In the unactuated state, the first valve unit  330  assumes a closed state, so that no hydraulic fluid can flow from the first brake circuit  10  into the simulator circuit  145 . In particular, the first valve unit remains closed in push-through operation of the assembly  100 , which prevents hydraulic fluid displaced from the main brake cylinder  110  from being able to flow into the simulator circuit  145 . 
         [0076]    The first valve unit  330  can be switched by electrical actuation via the ECU  200  to an open state, in order to connect the simulator circuit  145  fluidically to the first brake circuit  10 ,  20  and/or the main brake cylinder  110 . However, switching of the first valve unit  330  to an open state does not take place during normal brake operation. The opening of the first valve unit  330  takes place as part of an automatic ventilation method or as part of a test method for the brake system  1000 , but not during a braking process, as described further below in connection with  FIGS. 4-12B . In other words, the first valve unit  330  remains unactuated and thus closed during a braking process in normal operation of the assembly  100  also. 
         [0077]    The second valve unit  130  is in an open state in the unactuated state. The valve unit is open, for example, in push-through operation of the assembly  100 . In this case hydraulic fluid can drain from the hydraulic cylinder  120  into the unpressurised hydraulic fluid reservoir  170 . In normal operation of the assembly  100 , the valve unit  130  can be switched to a closed state by electrical activation via the ECU  200 . The simulator circuit  145  is disconnected hydraulically from the hydraulic fluid reservoir  170  by this. A hydraulic fluid displaced from the hydraulic cylinder  120  by actuation of the piston  122  is then conducted away via the fluid path  141  to the hydraulic pressure accumulator  144 , which then produces a realistic pedal response, as already discussed above. However, the valve unit  130  can also be actuated as part of a brake system test method or as part of an automatic ventilation method of the brake system  1000 , as described further below with reference to  FIGS. 4-12B . 
         [0078]    Overall the first fluid path  340  described here with the first valve unit  330  arranged in it and the second fluid path  140  with the second valve unit  130  arranged in it facilitate the realisation of a fluid circuit (termed test circuit below), which depending on the switching of the two valve units  130 ,  330  facilitates a fluidic connection of the main brake cylinder  110  to the simulator circuit  145  and a fluidic connection of the simulator circuit  145  to the unpressurised hydraulic fluid reservoir  170 . In this way, on opening of the first valve unit  330  and of the second valve unit  130 , a hydraulic fluid conveyed by the hydraulic pressure generator assembly  100  can flow via the first brake circuit  10  and via the first fluid path  340  connected fluidically to it into the simulator circuit  145 . From there the hydraulic fluid can flow back via the second fluid path  140  into the unpressurised hydraulic fluid reservoir  170  and into the main brake cylinder  110  (the unpressurised hydraulic fluid reservoir  170  is connected fluidically in a known manner to the main brake cylinder  110 ). As will be described in greater detail below, automatic ventilation of the brake system  1000  (or of a part of the brake system) or different automatic test methods for testing the simulator circuit and/or other components of the brake system  1000  can be implemented by the fluid circuit realised by the first fluid path  340 , the simulator circuit  145 , the second fluid path  140 . 
         [0079]      FIG. 1B  shows a brake system  1000   a  with an alternative configuration of a hydraulic pressure generator assembly  100   a . The assembly  100   a  differs from the assembly  100  shown in  FIG. 1A  substantially in that a cylinder-piston device  701 ,  702  is provided in addition to a main brake cylinder  110 ′ to generate hydraulic pressure, which device is connected fluidically on the outlet side to the main brake cylinder  110  via a fluid path  704  and is connected fluidically on the inlet side to a hydraulic fluid reservoir  170  via a fluid path  703 . All other components of the assembly  100   a , such as the electromechanical actuator  160 , the pedal interface  115  and the power transmission device  150 , for example, are substantially identical in their design and function to the assembly  100  shown in  FIG. 1A . They are therefore identified by the same reference numbers and are not described again. The same applies to the other components of the brake system shown in  FIG. 1B , such as the simulator circuit  145 , the ECU  300 , the HCU  300  with the brake circuits  10 ,  20  connected fluidically to the main brake cylinder  110 , the return lines  30 ,  40  and the hydraulic fluid reservoir  170 . The construction, arrangement and function of these components correspond to the construction, function and arrangement of the corresponding components in the brake system  1000  described in connection with  FIG. 1A . Reference is made here to the corresponding text passages of the description of  FIG. 1A . 
         [0080]    The fluidic connection of the cylinder-piston device  701 ,  702  to the main brake cylinder  110 ′ is described briefly below. The cylinder-piston device  701 ,  702  is connected fluidically on the outlet side via the fluid path  704  to a hydraulic chamber  111  defined by a rear side of a primary piston  112 ′ (right end face of the primary piston  112 ′ in  FIG. 1B ) and by a cylinder wall. The fluid line  704  is designed to transmit a hydraulic pressure generated in the cylinder-piston device  701 ,  702  to the primary piston  112 ′. The primary piston  112  (and consequently also a secondary piston  114 ′ arranged after the primary piston  112 ′) is then displaced (to the left in  FIG. 1B ) in consequence of a hydraulic pressure produced in the cylinder-piston device  701 ,  702  and present at the rear side of the primary piston  112 . A hydraulic pressure can be built up in this way with the aid of the cylinder-piston device  701 ,  720  via the main brake cylinder  110 ′ at the wheel brakes  401 - 404  of the brake system  1000   a . The hydraulic pressure built up at the wheel brakes  401 - 404  corresponds in this case to the hydraulic pressure produced in the cylinder-piston device  701 ,  702 . 
         [0081]    It remains to be stated that in the present arrangement, in normal operation of the brake system  1000   a , the primary piston  112 ′ of the main brake cylinder  110 ′ is actuated hydraulically by means of a separate pressure generator (cylinder-piston device  700 ,  701 ). In the configuration shown in  FIG. 1A , on the other hand, in normal operation of the brake system  1000 , the primary piston  112  is actuated by means of the electromechanical actuator  160  directly connecting to the primary piston  112 . Furthermore, in the arrangement shown in  FIG. 1B , the additional cylinder-piston device  701 ,  702  does not have any fluidic connection with one of the two brake circuits  10 ,  20  of the brake system  1000   a . Thus in normal operation the hydraulic pressure produced in the cylinder-piston device  701 ,  702  is exclusively available for actuation of the pistons  112 ,  114  of the main brake cylinder  110 . 
         [0082]    In respect of the arrangement and actuation of the first valve unit  330  in the first fluid path  340  and of the arrangement and actuation of the second valve unit  130  in the other fluid path  140 , let reference be made to the description of the first valve unit  330  and the second valve unit  130  of the brake system shown in  FIG. 1A . The only difference from the arrangement shown in  FIG. 1A  consists in the fact that the second fluid path  340  is connected fluidically by its end facing away from the simulator circuit  145  not to one of the brake circuits  10 ,  20  of the brake system, but either directly to the cylinder-piston device  701 ,  702  or to the hydraulic chamber  111  (as shown in  FIG. 1B ) of the main brake cylinder  111 ′. It is common to both arrangements that the first fluid path  340  facilitates a direct fluidic connection of simulator circuit  145  and the pressure-producing cylinder-piston device  701 ,  702 ,  110 ,  112 ,  114 . 
         [0083]    Reference is now made to the brake system  1000   b  shown in  FIG. 2 . With reference to  FIG. 2 , the construction and mode of functioning of the first valve unit  330  and of the second valve unit  130  as well as the valves arranged in the brake circuit  10 ,  20  and in the return lines  30 ,  40  are described. The construction and mode of functioning of the assembly  100  corresponds to that in  FIG. 1A  and will not therefore be described again. In particular, components of the hydraulic vehicle brake system  1000   b  shown in  FIG. 2  that correspond to those components shown in  FIG. 1A  or are similar in function to these are provided with the same reference numbers. Let reference be made in this connection to the description of  FIG. 1A . 
         [0084]    As shown in  FIG. 2 , the brake system  1000   b  (and the HCU  300 ) comprises a first group of four electrically actuatable valves  301 - 304  (termed third valve unit below), wherein each wheel brake  401 - 404  is assigned just one valve  301 - 304  of the third valve unit. The valve  301 - 304  assigned to each wheel brake  401 - 404  is designed to disconnect the wheel brake  401 - 404 , depending on the switching state of the valve  301 - 304 , hydraulically from the main brake cylinder  110  or to connect it hydraulically to the main brake cylinder  110 . The time activation of the individual valves  301 - 304  takes place in this case via the ECU  200 . 
         [0085]    For example, the valves  301 - 304  can be actuated by the ECU  200  in a time multiplex operation. Each valve  301 - 304  (and thus each wheel brake  401 - 404 ) can be assigned at least one time slot for a valve actuation. This assignment does not rule out individual valves  301 - 304  being kept open or closed over several time slots or more than two valves being open at the same time. In this way the hydraulic pressure built up by the assembly  100  at the wheel brakes  401 - 404  can be adjusted individual to the wheel or individual to the group of wheels for the purpose of vehicle dynamics control (thus e.g. in ABS and/or ASR and/or ESP control mode) in operational braking (if the assembly  100  is in normal operation). 
         [0086]    The brake system  1000   b  also comprises a second group of four electrically actuatable valves  311 - 314 , wherein each wheel brake is assigned just one valve  311 - 314 . The valves  311 - 314  are arranged in this case in the return lines of the wheel brakes  401 - 404 , wherein the return lines of the wheel brakes  401 - 404  of a brake circuit  10 ,  20  lead at the valve outlet of the valves  311 - 314  into the return line  30 ,  40  assigned to the brake circuit  10 ,  20 . The return lines  30 ,  40  lead into the hydraulic fluid reservoir  170 . The two valves  311 - 314  each assume a closed valve position in the unactuated state, so that no hydraulic fluid can flow from the respective wheel brakes  401 - 404  into the unpressurised hydraulic fluid reservoir  170 . As part of vehicle dynamics control (e.g. ABS and/or ASR and/or ESP control mode) they can be switched by electric activation of the ECU  200  to an open valve position, in order to allow hydraulic fluid to drain in a controlled manner via the respective brake circuits  10 ,  20  into the unpressurised hydraulic fluid reservoir  170 . 
         [0087]    The two brake circuits  10 ,  20  as well as the return lines  30 ,  40  assigned to the two brake circuits  10 ,  20  can each be connected fluidically to one another via a non-return valve  31 ,  41 . The non-return valves  31 ,  41  are arranged here, seen from the main brake cylinder  110 , ahead of the valves  301 - 304 ,  311 - 314  in a fluid path connecting the first brake circuit  10  to the first return line  40  and in a fluid path connecting the second brake circuit  20  to the second return line  30 . The two non-return valves  31 ,  41  are arranged in such a way that they do not let any hydraulic fluid flow from the respective brake circuit  10 ,  20  into the respective return line  30 ,  40 . On the other hand, however, hydraulic fluid can flow directly from the hydraulic fluid reservoir  170  via the non-return valves into the two chambers  116 ,  118  of the main brake cylinder  110 . This can be the case, for example, if the piston arrangement  112 ,  114  is in the reverse stroke and a vacuum is created in the chambers  116 ,  118 . It can be ensured in this way that the chambers  116 ,  118  of the main brake cylinder  110  are supplied with sufficient hydraulic fluid even after an actuation. 
         [0088]    Following a description of the valves  31 ,  41 ,  301 - 304 ,  311 - 314 , there now follows the description of the valves arranged in the first fluid path  340  and the second fluid path  140 . 
         [0089]    According to the embodiment of the brake system  1000   b  shown in  FIG. 2 , the first end of the first fluid path  340  leads into the first brake circuit  10  and the second end into the hydraulic pressure accumulator  144  of the simulator circuit  145 . Furthermore, the first end of the second fluid path  140  leads into the hydraulic fluid reservoir  170  (the hydraulic fluid reservoir  170  is not shown in  FIG. 2  and the opening of the second fluid path is only indicated schematically) and the second end into the hydraulic cylinder  120  of the pedal interface  115 . According to an alternative configuration, the first fluid path  340  and/or the second fluid path  140  can lead with its respective second end into the fluid path  141  of the simulator circuit  145  also. 
         [0090]    The first valve unit  330  arranged in the first fluid path  340  is realised in the embodiment of the brake system  1000   b  shown in  FIG. 2  as an electrically actuatable valve  330 . The valve  330  assumes a closed valve position in the unactuated state and thus blocks the first fluid path  340 , so that no hydraulic fluid can get from the main brake cylinder  110  or from the first brake circuit  10  into the simulator circuit  145  via the first fluid path  340 . The valve  330  remains closed during normal operation and during push-through operation of the assembly  100 . It is actuated as part of automatic test methods or as part of an automatic ventilation method, as described in connection with  FIGS. 4-12 . 
         [0091]    The second valve unit  130  arranged in the second fluid path  140  is realised in the embodiment of the brake system  1000   b  shown in  FIG. 2  as an electrically actuatable valve  132 . Connected downstream of the electrically actuatable valve  132  in the second fluid path  140  in the brake system  1000   b  shown in  FIG. 2  are also a first pressure relief valve  134  and a second pressure relief valve  136 . More precisely, the second fluid path  140  divides after the electrically actuatable valve  132  into a first branch  140   a  and a second branch  140   b , wherein the first branch  140   a  leads into the first brake circuit  10  and the second branch  140   b  leads into the unpressurised hydraulic fluid reservoir  170  or into the return line  40  leading to the unpressurised hydraulic fluid reservoir  170 . The first pressure relief valve  134  is arranged in the first branch  140   a  of the second fluid path  140 . The second pressure relief valve  136  is arranged in the second branch  140   b  of the second fluid path  140 . A third pressure relief valve  138  is also arranged in the brake system  1000   b  in parallel to the electrically actuatable valve  132  and to the second non-return valve  136 . 
         [0092]    The functions of the valves  132 ,  134 ,  136 ,  138  arranged in the second fluid path are described in greater detail below. 
         [0093]    The electrically actuatable valve  132  arranged in the second fluid path  140  is formed to disconnect or connect the hydraulic cylinder  120  and the simulator circuit  145  fluidically from/to the unpressurised hydraulic fluid reservoir  170 . Depending on the operating mode of the brake system (push-through operation, brake-by-wire or normal operation, test operation or ventilation of the brake system), the electrically actuatable valve  132  can be switched to an open or closed valve position. In the unactuated state (de-energised state) the electrically actuatable valve  132  assumes an open valve position. In push-through braking operation the valve  132  remains unactuated and thus functionless. Hydraulic fluid displaced from the hydraulic cylinder  120  in push-through operation can then flow via the open valve  132  to the first and second pressure relief valves  134 ,  136  arranged downstream and via these valves (depending on the hydraulic pressure in the first brake circuit  10 ) either flow into the first brake circuit  10  or drain into the hydraulic fluid reservoir  170 . 
         [0094]    In a braking process in normal operation of the assembly  100 , on the other hand, the valve  132  is switched under current control to a closed valve position. No function is then assigned to the first pressure relief valve  134  and the second pressure relief valve  136 , as they are completely fluidically disconnected by the valve  132  from the hydraulic cylinder  120 . The first fluid path  140  is blocked by the electrically actuatable valve  132  and hydraulic fluid can neither flow from the hydraulic cylinder  120  of the pedal interface  115  into the main brake cylinder  110  and/or the unpressurised hydraulic fluid reservoir  170 , nor in the reverse direction from the main brake cylinder  110  and/or the unpressurised hydraulic fluid reservoir  170  into the hydraulic cylinder  120 . On the contrary, the hydraulic fluid displaced from the chamber  124  of the hydraulic cylinder  120  is conveyed via the throttle valve  143  into the hydraulic pressure accumulator  144 , wherein the hydraulic pressure accumulator  144  simulates the feedback described above. On the return movement of the brake pedal  126 , the valve  132  can pass once again into the open valve position in that the power supply for the valve  132  is interrupted. 
         [0095]    The actuation of the valve  132  as part of a brake system test method or as part of automatic ventilation of the brake system  1000   b  is described in connection with  FIGS. 4-12B . 
         [0096]    The first pressure relief valve  134  and the second pressure relief valve  136  are provided for the pressure-controlled feed of additional hydraulic fluid from the hydraulic cylinder  120  into the at least one brake circuit  10 ,  20  of the brake system  1000   b  in push-through operation of the assembly  100 . 
         [0097]    The first pressure relief valve  134  is formed in the shape of a non-return valve. The non-return valve  134  is arranged so that in the open valve position it only allows hydraulic fluid to flow from the hydraulic cylinder  120  into the first brake circuit  10 , but blocks it absolutely in the reverse direction. The first non-return valve  134  is formed as a spring-loaded non-return valve, which is limited to an overflow pressure of 0.3 bar. Thus in push-through operation, hydraulic fluid from the hydraulic cylinder  120  can always be fed via the valve  132  (this is open in push-through operation) and the non-return valve  134  connected in series into the first brake circuit (and via the main brake cylinder  110  connected fluidically to it also into the second brake circuit) if the hydraulic pressure produced by displacement of the piston  122  in the hydraulic cylinder  120  (which pressure is consequently also present at the valve inlet of the first non-return valve  134 ) is at least 0.3 bar higher than the hydraulic pressure produced in the main brake cylinder  110 , which pressure is also present at the valve outlet of the first non-return valve  134 . Furthermore, hydraulic fluid is pumped into the main brake cylinder  110  and into the two brake circuits  10 ,  20  (as long as the hydraulic pressure generated in the main brake cylinder  110  is still small) not only at the beginning of the push-through phase, but also during a pressure build-up phase. Overall, hydraulic fluid displaced from the hydraulic cylinder  120  in push-through operation is thus fed into the two brake circuits  10 ,  20  under pressure. 
         [0098]    The additional hydraulic fluid fed into the brake circuits  10 ,  20  helps to overcome the clearance of the wheel brakes  401 - 404  quickly without much hydraulic fluid having to be conveyed from the main brake cylinder  110  into the two brake circuits  10 ,  20  for this. Due to the additional hydraulic fluid supplied, the actuation path of the piston arrangement  112 ,  114  (and thus of the brake pedal  126 ) is considerably shortened, especially in the initial push-through phase. This is because the initial actuation path (filling path) of the piston arrangement  112 ,  114 , which is only necessary to displace hydraulic fluid from the main brake cylinder  110  to overcome the clearance at the wheel brakes  401 - 404 , can be compensated for partially or completely by the additional fluid feed from the hydraulic cylinder  120 . 
         [0099]    In push-through operation, the piston  122  and the piston arrangement  112 ,  114  are actuated via the power transmission device  150  only by the actuation force applied to the pedal  126 . Thus the feed pressure present at the first pressure relief valve  134  and the hydraulic pressure produced on account of the actuation of the piston arrangement  112 ,  114  react directly via the power transmission device  150  directly on the piston  122  and on the pedal  126  connected to it. In particular, to prevent a large part of the actuation force applied to the brake pedal  126  from being consumed for the generation of a feed pressure and thus no longer being available for the pressure build-up in the main brake cylinder  1110 , the second pressure relief valve  136  is provided. This is arranged in the second branch  140   b , which leads into the unpressurised hydraulic fluid reservoir  170 . The second pressure relief valve  136  is arranged such that its valve inlet is connected fluidically to the valve inlet of the first pressure relief valve  134 , while its valve outlet is connected fluidically to the unpressurised hydraulic fluid reservoir  170 . 
         [0100]    According to the variant shown in  FIG. 2 , the second pressure relief valve  136  is formed as a pressure-controlled pressure relief valve  136 , which, upon a predetermined pressure being attained in the main brake cylinder  110  (for example 10 bar), switches from a closed state to an open state. On attaining the predetermined pressure in the main brake cylinder  110 , the pressure relief valve  136  switches to an open valve position. The hydraulic fluid dammed up in the fluid path  140  and at the valve inlets of the valves  134 ,  136  during the push-through operation can then flow away without pressure into the unpressurised hydraulic fluid reservoir  170  via the second partial path  140   b.    
         [0101]    The construction and the mode of functioning of the pressure-controlled second pressure relief valve  136   b  are described in greater detail with reference to  FIGS. 3 a    and  3   b.    
         [0102]    The pressure relief valve  136  has a valve inlet  601 , a valve outlet  609  and a pressure inlet  602 . The valve inlet  601  is connected fluidically to the hydraulic cylinder  120 . The valve outlet  609  is connected fluidically to the unpressurised hydraulic fluid reservoir  170 . Furthermore, the pressure inlet  602  has a fluidic connection to the main brake cylinder  110 . The pressure inlet  602  leads into a first valve hole  604 , which takes up a first spring element  606  and a piston  603  with a seal element  605  displaceably. The spring element  606  is designed to pretension the piston  603  in the direction of the pressure inlet  602 . The valve inlet  601  leads into the first hole  604  at its end facing away from the pressure inlet  602 . The piston  603  has a corresponding first fluid passage  615  in the area of the valve inlet  601  in order not to block the valve inlet  601  on actuation of the piston. The first seal element  604  is arranged laterally between the piston  603  and the valve inner wall  607  in such a way that it separates the pressure inlet  602  from the valve inlet  601  fluidically at all times (thus in the unactuated and in the actuated valve state). 
         [0103]    At its end lying opposite the pressure inlet  602 , the first hole  604  leads into a second hole  608 . The second hole  608  takes up a stopper-shaped valve element  613 , which has a second fluid passage  614  and a hole for leading a valve tappet  611  through. The valve tappet  611  is connected at its end facing the valve outlet  609  to a closing element  612 , which can be brought into contact with an end face of the stopper  613  facing the valve outlet. The valve tappet  611  is also provided at its end facing away from the valve outlet  601  with a spring bearing element  610 . A second spring element  616  fixed between the spring bearing element  610  and a rear side of the stopper  613  brings the tappet  611  into contact with the piston  602 . At the same time, the closing element  612  is pressed onto the stopper  613 , due to which the second fluid passage  614  is blocked. 
         [0104]    In the unactuated state ( FIG. 3A ), no hydraulic fluid can pass from the valve inlet  601  to the valve outlet  609 , as the second fluid passage  614  is blocked by the closing element  612  that is in contact with the stopper  613 . The hydraulic fluid dammed up at the valve inlet  601  and at the second passage  609  cannot lift the closing element  612  and release the passage. On the contrary, the fluid flows via the first non-return valve  134  into the main brake cylinder  110  and into the brake circuit  10 . 
         [0105]    The pressure produced in the main brake cylinder  110  acts directly on the piston  603 , as this is connected fluidically via the pressure inlet  602  to the main brake cylinder  110  (and to the first brake circuit  10 ). If the pressure produced in the main brake cylinder  110  is sufficiently great, so that the force acting on the piston  603  (this is proportional to the pressure present and the area of piston exposed to the pressure) exceeds the spring force of the first and second spring device  606 ,  615 , then the piston  602  and the valve tappet  611  brought into contact with the piston are displaced in the direction of the valve outlet (to the left in  FIG. 3B ). The second fluid passage  614  is thus released and hydraulic fluid can now flow via the first fluid passage  615  and the second fluid passage  614  free of pressure into the unpressurised hydraulic fluid reservoir  170  (cf. arrow in  FIG. 3B , which marks the fluid path). The valve  136   b  is switched from a closed to an open valve position solely by the pressure produced in the brake circuit  10 . 
         [0106]    Overall the pressure-controlled valve  136  functions as a control valve in push-through operation, which determines for how long hydraulic fluid is fed from the hydraulic cylinder  120  into the brake circuit  10 . In other words, the filling phase in push-through operation of the brake system can be determined via the pressure-controlled valve  136 . The valve  136  also prevents an unnecessary damming up of hydraulic fluid at the first non-return valve  134 , if, for example, the pressure produced in the main brake cylinder  110  during the push-through phase approaches or even exceeds the pressure produced in the hydraulic cylinder  120 . 
         [0107]    Back to  FIG. 2 . The third non-return valve  138  arranged in parallel to the second pressure relief valve  136  and to the electrically actuatable valve  132  is arranged in such a way that its valve inlet is connected fluidically to the valve outlet of the second non-return valve  136  and its valve outlet to the valve inlet of the electrically actuatable valve  132 . 
         [0108]    The third non-return valve  138  is limited to an overpressure of approximately 0.4 bar. It is formed to enable hydraulic fluid to flow back from the unpressurised hydraulic fluid reservoir  170  into the hydraulic cylinder  120  on the return movement of the brake pedal  126  (and thus on a return stroke of the piston  122 ). On the return stroke of the piston  120 , a negative pressure can arise in the fluid path  140  and in the simulator circuit  145  relative to the unpressurised hydraulic fluid reservoir  170 . Due to the arrangement of the third non-return valve  138 , hydraulic fluid can flow past the electric valve  132  directly into the fluid path  140  in the return stroke. Since the third non-return valve  138  has a large valve cross section compared with the electrically actuatable valve  132 , hydraulic fluid can be returned swiftly to the hydraulic cylinder  120  during a return stroke movement, in order to fill the hydraulic cylinder  120  with hydraulic fluid again. In particular, pumping of the hydraulic pressure accumulator  144  in the event of several short brake operations following on from one another quickly can thus be avoided in push-through operation. An exchange of hydraulic fluid between the unpressurised hydraulic fluid reservoir  170 , the hydraulic cylinder  120  and the simulator circuit  145  connected fluidically to the hydraulic cylinder  120  can also take place via the third non-return valve  138  during normal operation of the brake system  1000 . 
         [0109]    Independently of the pressure-controlled valves  134 ,  136 ,  138  arranged in the second fluid path  140 , a fluid circuit can be realised by the first fluid path  340  and the valve  330  arranged therein and the second fluid path  140  and the electrically actuatable valve  132  arranged therein (this is connected fluidically to the valve  330  only via the simulator circuit), which circuit, starting from the main brake cylinder  110 , extends via the first fluid path  340 , via the simulator circuit  145  and via the second fluid path  130  to the hydraulic fluid reservoir  170 . This fluid circuit forms a “test circuit” for the brake system  1000 , which can be used for automatic checking of the simulator circuit  145  and/or for checking the pressure valve arranged in the test circuit. The test circuit can also be used for automatic ventilation of the brake system  1000   b  or at least for automatic ventilation of parts of the brake system  1000   b , as described in greater detail with reference to the following  FIGS. 4-12B . 
         [0110]    The brake system  1000   b  shown in  FIG. 2  also comprises a sensor  2000  arranged in the main brake cylinder  110  or in the first brake circuit  10  for detecting the pressure in the main brake cylinder  110  or in the brake circuit  10 , a sensor  2002  arranged in the simulator circuit  145  for determining the pressure in the simulator circuit  145 , a path and/or force sensor  2004  arranged in the pedal interface  115  for determining the brake pedal actuation in brake operation and at least one sensor  2006  arranged on the actuator  160  for determining the actuator operation. The sensor  2006  can detect a motor parameter, such as the motor position during the operation. The actuation (thus displacement) of the primary piston  112  can be detected from the motor position and the known transmission ratio of the nut-spindle arrangement  162 ,  163  connected in series to the motor  161 . Alternatively to this, a path sensor can also be present for the direct measurement of the spindle displacement in the event of an actuator operation, in order to detect from this the displacement of the primary piston  112  and of the secondary piston  114  initiated by the actuator  160 . The volume of hydraulic fluid displaced from the main brake cylinder  110  can be calculated in turn from the displacement of the piston arrangement  112 ,  114 . 
         [0111]    An automatic test method for checking the functionality of the electrohydraulic vehicle brake systems  1000 ,  1000   a ,  1000   b  shown in  FIGS. 1A / 1 B and  2  is now described with reference to  FIGS. 4 and 5 . Specifically, a test method for checking the simulator circuit  145  is described.  FIG. 4  shows a flow chart, which clarifies the sequence of the method.  FIG. 5  shows, with reference to the brake system  1000   b  illustrated in  FIG. 2 , the valve positions and the flow conditions of the hydraulic fluid in the brake system  1000   b.    
         [0112]    The automatic test method is implemented with the aid of the test circuit described above. The test method can be lodged with reference to control routines in the ECU  200 , which comprises the sequential steps of activation of the valves and the operation of the electromechanical actuator  160 . 
         [0113]    The method is executed during a brake-operation-free phase of the brake system  1000 ,  1000   a ,  1000   b . The valves  130 ,  301 - 304 ,  330  of the first, second and third valve unit are initially still unactuated in this configuration. Specifically the valve  330  of the first valve unit is in a closed valve position, the valve  130  of the second valve unit in an open valve position and the valves  301 - 304  of the third valve unit each in an open valve position, as illustrated in  FIG. 2 . 
         [0114]    In a first step S 10 , the valve  330 , which is arranged in the first fluid path  340 , is first actuated electrically and thus switched from a closed valve state to an open valve state. In this way the pedal simulator circuit  145  and the pedal interface  115  are connected fluidically to the hydraulic pressure generator of the brake system ( 1000 ,  1000   a ,  1000   b ). In the embodiments described here of  FIGS. 1A and 1B , the hydraulic pressure generator is realised by the main brake cylinder  110 , the piston arrangement  112 ,  114  taken up in the main brake cylinder  110  and by the actuator  160  ( FIG. 1A ) acting on the primary piston  112  or by the cylinder-piston device  701 ,  702  and the actuator  160  ( FIG. 1B ) acting on the cylinder-piston device  701 ,  702 . Alternatively the hydraulic pressure generator can also be realised by a cylinder-piston system such as described in WO 2011/141158 A2, for example. 
         [0115]    In a second step S 20 , the electrically actuatable valve  130  arranged in the second fluid path  130  is also switched from an open valve position to a closed valve position. The simulator circuit  145  and the pedal interface  150  are disconnected fluidically from the hydraulic fluid reservoir  170  by this. 
         [0116]    If the valves  301 - 304  of the third valve unit are in an open state, these are switched respectively from an open valve position to a closed valve position in a third step S 30 . The wheel brakes  401 - 404  are thus disconnected fluidically from the main brake cylinder  110 , so that on operation of the actuator  160  no hydraulic fluid can flow into the wheel brakes  401 - 404 . The wheel brakes  401 - 404  remain disconnected fluidically from the main brake cylinder  110  during the test method. 
         [0117]    The actuation steps S 10  to S 30  described here can take place simultaneously or consecutively in the order described above or can be executed in another order. The actuation described of the valves  130 ,  301 - 304 ,  330  is also illustrated in  FIG. 5 . 
         [0118]    In a subsequent fourth step S 40 , the electromechanical actuator  160  is now operated to displace hydraulic fluid from the hydraulic pressure generator into the simulator circuit  145 . 
         [0119]    In the brake system shown in  FIGS. 1A and 2 , the electromechanical actuator  160  moves the primary piston  112  and the secondary piston  114  connected to it in the main brake cylinder  110  in the travel direction (cf. displacement to left in  FIG. 5 ), due to which hydraulic fluid can be displaced from the two chambers  116 ,  118  into the brake circuits  10 ,  20 . The flow of the hydraulic fluid displaced from the main brake cylinder  110  in the brake system  1000   b  is represented in  FIG. 5  by the thickly marked fluid paths. The hydraulic fluid displaced from the main brake cylinder  110  during actuator operation cannot get to the wheel brakes  401 - 404  on account of the closed valves  301 - 304 . It flows instead via the first brake circuit  10  and via the first fluid path  340  into the hydraulic pressure accumulator  144 , due to which this is filled with the hydraulic fluid conveyed from the main brake cylinder  110 . Since the valve  130  in the second fluid path  140  is closed, no fluid can flow via the second fluid path  140  into the hydraulic fluid reservoir  170  either. 
         [0120]    Simultaneously with the operation of the electromechanical actuator  160 , the hydraulic pressure present in the main brake cylinder  110  or in the cylinder  701  is detected at the main brake cylinder  110  via the sensor  2000  (fifth step S 50 ). In the ideal case, if the simulator circuit  145  does not contain any compressible air, the hydraulic pressure built up in the hydraulic pressure generator during the actuator operation corresponds to the counterpressure produced by displacement of the pretensioned piston in the hydraulic pressure accumulator  144 . The pressure characteristic of the hydraulic pressure accumulator  144  can thus be checked. If there is air in the simulator circuit  145 , the pressure rise can deviate from an expected pressure rise on account of the compressibility of the air. The deviation is a measure in this case of the level of ventilation of the simulator circuit  145 . 
         [0121]    To be able to test the simulator circuit  145 , the hydraulic fluid displaced from the hydraulic pressure generator during the actuation of the pistons  112 ,  114 ,  702  is also detected by the sensor  2006  in a further sixth step S 60 . The detection of the fluid volume takes place in this case substantially simultaneously with the detection of the hydraulic pressure. The detection of the displaced fluid volume can be determined from a detected motor position, from the transmission ratio of the nut-spindle gear and from the cylinder diameter of the hydraulic pressure generator. Alternatively, the actuation path of the piston  112 ,  702  of the hydraulic pressure generator can be measured directly by a path sensor in the hydraulic pressure generator or by a path sensor mounted in the nut-spindle gear and the volume of fluid displaced can be determined from the actuation path and the cylinder diameter of the hydraulic pressure generator (from which the cylinder base of the hydraulic pressure generator can be determined in a known manner). The detection of pressure and displaced volume can take place continuously or incrementally at set time intervals. The pressure rise can be determined as a function of the fluid volume conveyed in the simulator circuit  145  from the detected measured value tuples “pressure” and “fluid volume”. 
         [0122]    The pressure-volume characteristic thus obtained can then be compared in a final seventh step S 70  with a stored pressure-volume characteristic. The degree of ventilation of the simulator circuit  145  can be determined with reference to possible deviations of the detected pressure-volume characteristic from the stored characteristic. If the pressure rise on displacement of a hydraulic fluid volume during the test lies behind the pressure rise to be expected, for example, the proportion of air in the simulator circuit and thus the degree of ventilation of the simulator circuit  145  can be determined from this. Depending on the result, ventilation of the simulator circuit can then be carried out. A ventilation method is described below in connection with  FIGS. 9 and 12B . 
         [0123]    As soon as it is ensured that the simulator circuit  145  has been sufficiently ventilated, the piston-spring arrangement in the hydraulic fluid accumulator  144  can also be tested itself. The counterpressure detected in the hydraulic pressure generator is proportional to the spring force and to the displacement path of the spring-piston arrangement. Deviations in the pressure-volume characteristic can point to a reduced spring force and thus to signs of wear of the hydraulic pressure accumulator  144 . 
         [0124]    If a detected fluid volume has first been conveyed into the simulator circuit  145  as part of the simulator circuit test described above, the flow properties of the electrically actuatable valve  132  arranged in the second fluid path  140  can be tested in a further test. The corresponding flow chart is shown in  FIG. 6 . 
         [0125]    The method for testing the flow properties of the valve  132  can take place following the simulator circuit test. In this connection the valve  132  is switched to an open valve position in an eighth step S 80 . The hydraulic fluid dammed up in the simulator circuit  145  can then flow via the hydraulic cylinder  120  of the unactuated pedal interface  115  that is connected fluidically to the simulator circuit  145 , the second fluid path  140  and via the open valve  132  into the unpressurised hydraulic fluid reservoir  170 . Since the hydraulic pressure built up in the hydraulic pressure generator for complete filling of the hydraulic pressure accumulator  144  is substantially higher than the switching pressure of 10 bar needed to release the pressure-controlled pressure relief valve  136 , the pressure-controlled pressure relief valve  136  arranged downstream of the electrically actuatable valve  132  is in an open position, so that the fluid can flow into the hydraulic fluid reservoir  170 . 
         [0126]    During the return flow of the fluid into the hydraulic fluid reservoir  170 , the falling hydraulic pressure in the simulator circuit  145  or in the hydraulic pressure generator is detected (step S 90 ). Since the hydraulic fluid volume displaced into the simulator circuit  145  is known, it can be determined from the detection in time of the pressure drop in the main brake cylinder  110  how much fluid flows via the valve  132  per unit of time. In other words, the measured fall in time of the pressure (pressure drop rate) is a measure of how great a volume of fluid flows per unit of time through the valve cross section of the valve  132 . The functionality of the valve  132  in the second fluid path  140  can be checked in this way. 
         [0127]    The test method for determining the flow properties of the valve  132  can also be executed independently of the simulator circuit test. In this case, the valves  130 ,  301 - 304 ,  340  of the first, second and third valve unit are actuated, as described in the steps S 10 , S 20 , S 30 . Then the actuator  160  is operated, in order to convey a fluid volume into the simulator circuit  145  (step S 40 ). The fluid volume conveyed is determined according to step S 60 . Then the valve  132  is opened and the pressure drop in the hydraulic pressure generator detected (steps S 80  and S 90 ). 
         [0128]    Another test method for determining the switching pressure of the pressure-controlled pressure relief valve  136  is now explained with the aid of  FIGS. 7 and 8 .  FIG. 7  shows a flow chart that illustrates the method steps of the test method.  FIG. 8  shows the switching of the valves  130 ,  301 - 304 ,  330  and the flow path with reference to the brake system  1000   b  illustrated in  FIG. 2 . 
         [0129]    As already described in connection with  FIG. 2 , the pressure-controlled pressure relief valve  136  and the non-return valve  134  are arranged in the second fluid path  140 , in order to facilitate a pressure-controlled feed of hydraulic fluid from the hydraulic cylinder  120  into the brake circuits  10 ,  20  of the brake system  1000   b  during push-through operation. 
         [0130]    To determine the switching pressure of the pressure-controlled pressure relief valve  136 , the valve  330  is switched from a closed valve position to an open valve position in a first step S 110 . The valve  132  remains unactuated and thus in an open valve position. If the valve  132  should be located in a closed valve position before execution of the test, the valve is switched to an open valve position. 
         [0131]    In a subsequent second step S 120 , the valves  301 - 304  of the third valve unit are also actuated and switched to a closed valve position. Thus the wheel brakes  401 - 404  are disconnected fluidically from the main brake cylinder  110  during the test. 
         [0132]    Disconnection of the wheel brakes  401 - 404  during the test is necessary so that no hydraulic fluid gets into the wheel brakes  401 - 404 . If hydraulic fluid were to reach the wheel brakes  401 - 404  additionally during the test, the switching pressure of the pressure relief valve  136  could not be determined reliably on account of the actuation of the wheel brakes  401 - 404  then taking place. The steps S 110 , S 120  can be executed simultaneously or according to a set time sequence. 
         [0133]    In a following third step S 130 , the electromechanical actuator  160  is then operated, which moves the piston arrangement  112 ,  114  in the main brake cylinder  110  or the piston  702  in the cylinder  701  in the direction of travel, in order to convey hydraulic fluid from the hydraulic pressure generator to the hydraulic fluid reservoir. 
         [0134]    In the brake system shown in  FIG. 2 , the hydraulic fluid conveyed on operation of the electromechanical actuator  160  passes first into the brake circuits  10 ,  20 . Due to the closed valves  301 - 304 , the hydraulic fluid displaced from the chambers  116 ,  118  can only be moved via the first fluid path  340  and the open valve  330  to the hydraulic pressure accumulator  144 . From the hydraulic pressure accumulator  144  the hydraulic fluid can pass into the second hydraulic fluid path  140  via the pedal interface  115 . The hydraulic fluid conveyed in the second hydraulic path  140  can also reach the inlets of the valves  134 ,  136  via the second valve  132 . In the present test method, approximately the same hydraulic pressure is present at the non-return valve  134  as in the main brake cylinder  110 . It thus remains closed. The pressure-controlled pressure relief valve  136  is initially likewise closed. The fluid conveyed from the main brake cylinder  110  thus dams up in the test circuit (first fluid path  340 , simulator circuit  145 , second fluid path  140 ). 
         [0135]    With increasing actuator operation, the pressure in the main brake cylinder and in the test circuit rises continuously. The temporal rise in the pressure is detected in a further step S 140  at the main brake cylinder by the sensor  2000  (or in the test circuit by the sensor  2002 ). 
         [0136]    The valve  136  switches to an open valve position (cf.  FIG. 8 ) precisely when the hydraulic pressure present in the main brake cylinder  110  reaches the switching pressure of the valve  136  (thus 10 bar). As soon as the pressure relief valve  136  passes from the closed valve position to the open valve position, the hydraulic fluid backed up at the inlets of the valves  134 ,  136  can flow via the open valve  136  and the partial path  140   b  into the fluid reservoir  170 . Following the opening of the valve  136 , upon further displacement of the piston arrangement  112 ,  114  the hydraulic pressure in the main brake cylinder  110  and in the simulator circuit  145  is increased only insignificantly. This levelling off of the pressure on reaching a certain pressure level in the main brake cylinder can be detected by the sensor  2000 . It is a measure of the switching pressure of the pressure-controlled pressure relief valve  136 . 
         [0137]    By recording the pressure during operation of the electromechanical actuator  160  (cf.  FIG. 8 , in which the pressure detection device  2000  is circled), the switching pressure at which the pressure-controlled pressure relief valve  136  switches from a closed valve position to an open valve position can thus be determined. In particular, the tightness of the pressure-controlled pressure relief valve  136  can also be tested by the described test. For example, in the case of a leaking valve  136 , hydraulic fluid can flow from the first brake circuit  10  or the main brake cylinder  110  via a control channel (dashed line at valve  136  in  FIG. 8 ) and via the valve  136  directly into the hydraulic fluid reservoir  170 , due to which a pressure build-up in the brake circuit  10  would be considerably obstructed. 
         [0138]    The test methods described here are each executed during a brake-operation-free phase. According to one implementation, the test methods described here can be executed when the vehicle is stationary. This can be the case, for example, if the vehicle is stopped or if the vehicle is in a stationary position (e.g. at a traffic light) during a journey. If a movement of the vehicle is detected in this connection and if a test method is just being executed, the test method is interrupted or aborted in order not to influence the operability of the brake system  1000   b . The test methods described also run entirely automatically. They can be repeated at regular intervals or also at the request of the driver. 
         [0139]    The test circuit described in connection with  FIGS. 1A, 1B and 2  can also be used to ventilate the brake system  1000 ,  1000   a ,  1000   b  or to ventilate parts of the brake system  1000 ,  1000   a ,  1000   b . The automatic ventilation of the brake system  1000 ,  1000   a ,  1000   b  or of the simulator circuit  145  shown in  FIGS. 1A and 2 , which can be executed with the aid of the hydraulic pressure generator assembly  100  independently of the driver and thus entirely automatically, is now described below with reference to two examples. 
         [0140]    A first embodiment of a ventilation of the simulator circuit  145  is shown in  FIGS. 9 and 10 . In  FIG. 9 , a corresponding flow chart of the automatic ventilation method is shown. In  FIG. 10  the valve switchings and the flow route of the hydraulic fluid with reference to the brake system  1000   b  shown in  FIG. 2  are shown during the ventilation method. 
         [0141]    In a first step S 210 , the third valve unit  301 - 302  is first switched to a closed valve position, in order to disconnect the wheel brakes  401 - 404  assigned to the valve units  301 - 304  hydraulically from the main brake cylinder  110  (or in the case of the hydraulic pressure generator assembly according to WO 2011/141158 A2 from the cylinder-piston arrangement for hydraulic pressure generation). 
         [0142]    In a subsequent second step S 220 , the valve  330  arranged in the first hydraulic fluid path  340  is opened to connect the main brake cylinder  110  fluidically to the simulator circuit  145 . Furthermore, the valve  132  arranged in the second hydraulic fluid path  140  is moved into an open valve position unless this valve is already in the open valve position. The switching of the valves  301 - 304 ,  340  and optionally of the valve  132  can take place simultaneously or consecutively in time by the ECU  200 . 
         [0143]    In a following third step S 230 , the electromechanical actuator  160  is now operated to convey hydraulic fluid from the hydraulic pressure generator. In the variant of the brake system shown in  FIG. 2 , the actuator  160  actuates the piston arrangement  112 ,  114  of the main brake cylinder  110 . By displacing the piston arrangement  112 ,  114  of the main brake cylinder  110 , hydraulic fluid can only flow from the first chamber  116  of the main brake cylinder  110  via the first brake circuit  10  and via the first fluid path  340  and via the open valve  330  into the simulator circuit  145 . With increasing displacement of the piston arrangement  112 ,  114 , the pressure in the test circuit and in the main brake cylinder  110  continues to rise. As soon as the switching pressure of the pressure-controlled pressure relief valve  136  (10 bar) is reached, the valve  136  switches to an open valve position  136 . The hydraulic fluid conveyed in the simulator circuit  145  can then flow via the fluid path  141  of the simulator circuit  145 , via the hydraulic cylinder  120  of the unactuated brake pedal interface  115  and via the open valves  132  and  136  in the second fluid path back into the hydraulic fluid reservoir  170  (and thus into the main brake cylinder  110  connected to the hydraulic fluid reservoir  170 ). New hydraulic fluid can be conveyed in this way into the simulator circuit  145  and into the pedal interface  115  without operation of the brake pedal  126  being required for this. 
         [0144]    According to an alternative ventilation variant, which is illustrated in  FIGS. 11, 12A and 12B , the hydraulic fluid takes the following flow route in the brake system  1000   b.    
         [0145]    First the valves  301 - 304  of the third valve unit are switched to an open valve position unless they are already in an open valve position (first step S 310 ). The valve  320  arranged in the first hydraulic fluid path  340  also remains unactuated initially and thus in a blocking position. 
         [0146]    Then the actuator  160  is operated (step S 320 ) to convey hydraulic fluid from the main brake cylinder  110  into the wheel brakes  401 - 404  of the two brake circuits  10 ,  20  (cf.  FIG. 12A , where the route of the displaced hydraulic fluid is represented by thick marking of the fluid paths). The hydraulic fluid displaced from the main brake cylinder  110  can initially only reach the wheel brakes  401 - 404 , which are actuated due to the hydraulic fluid conveyed. The valve  136  is already switched to an open position on account of the pressure present in the main brake cylinder  110 . The hydraulic fluid displaced into the wheel brakes  401 - 404  cannot yet flow to the hydraulic fluid reservoir  170 , however, since the valve  330  in the first fluid path  340  is still closed. 
         [0147]    The valve  330  is then switched to an open valve position (step  330 ) to allow the hydraulic fluid to flow from the wheel brakes  401 - 404  via the two brake circuits  10 ,  20  and via the first fluid path  340  into the hydraulic pressure accumulator  144  (cf.  FIG. 12B , the hydraulic pressure accumulator  144  is filled). From there the hydraulic fluid flows via the fluid path  141  and the hydraulic cylinder  120  of the unactuated pedal interface  115  to the second fluid path  140 . The hydraulic fluid displaced from the main brake cylinder  110  can flow back into the fluid reservoir  170  via the second fluid path  140  and via the open valves  132  and  136 . 
         [0148]    Overall a test circuit is realised by the brake system  1000 ,  1000   a ,  1000   b  described here that is provided for the automatic ventilation of the brake system  1000  and for the testing of valves in the brake system  1000 . The test circuit substantially consists of two hydraulic fluid paths different from one another (the first fluid path  340  and second fluid path  140  described above), which fluidically connect the simulator circuit  145  to the hydraulic fluid reservoir  170  on the one hand and to a hydraulic pressure generator (with the main brake cylinder  110  in the embodiments described here) on the other hand. The present test circuit design can easily be integrated into modern brake systems, which operate according to the brake-by-wire principle, as they often already have a hydraulic simulator circuit and a fluidic connection to the hydraulic fluid reservoir  170 . Only another fluid path having a valve unit has to be implemented in the brake system  1000 ,  1000   a ,  1000   b  in order to connect the hydraulic pressure generator of the brake system  1000 ,  1000   a ,  1000   b  fluidically to the simulator circuit. 
         [0149]    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.