BRAKE SYSTEM COMPRISING AT LEAST ONE CONVEYING UNIT FOR REDELIVERING BRAKE FLUID TO THE WORKING CHAMBERS OF A BRAKE BOOSTER

Abstract

The invention relates to a brake system comprising an electromotive drive device for a brake booster having at least one piston, the at least one piston being driven by the drive device for pressure build-up and pressure relief in at least one wheel brake and being mechanically adjustable by means of a brake actuator, especially in the form of a brake pedal, only in the event of malfunction of the drive device, at least one decoupling valve for closing and opening the brake line being arranged in the brake line between every wheel brake and a working chamber of the brake booster. The brake system is characterized in that at least one delivery unit delivers, when required, brake fluid from a reservoir to the respective working chamber of the brake booster via at least one supply line that is connected to a brake line or the brake booster, especially to a working chamber, a controlled valve for opening and closing the respective supply line being arranged in every supply line.

Description

The present invention relates to a brake system according to the preamble of claim 1.

In conventional systems the master brake cylinder (HZ) of an automotive brake system is adapted to have a volume of displacement that corresponds to the volume intake of the wheel brakes required for reaching the maximum brake pressure. Additionally, a reserve for extreme situations such as brake fading or poorly vented brake systems is provided. The volume intake of the wheel brakes depends on the vehicle. In principle, the volume increases relative to the weight of the vehicle. The displacement volume of the Hz can be determined by the diameter of its piston and its stroke. In conventional brake systems the brake pedal is connected to the Hz via the push rod piston (DK), the pedal travel being connected to the push rod piston (DK) of the Hz via a constant pedal ratio. For ergonomic reasons, the brake system is designed for a locking pressure to be reached at about 50% of pedal travel. For higher vehicle categories the required necessary displacement volume has to be provided by an increase of the diameter of the Hz since the pedal travel in the vehicle is limited.

The piston force required for achieving the desired brake pressure is a result of the piston surface area. The pedal force is boosted by a so-called vacuum brake booster (Vak. BKV) so that with an intact-brake power assistance the pedal forces will be moderate.

Two further facts are critical for the design of the brake system:

• • The boosting force of the Vak. BKV is limited by the so-called cut-out point. In principle, this cut-out point lies a little above the Locking pressure at a high friction value which corresponds to about 100 bar. However, in case of fading of the brake system higher pressures are required so as to enable the brake power assistance to reach saturation and the pedal forces to increase considerably. To counter this effect vehicles with an ESP system use the so-called ESP pump for pumping brake fluid from the HZ into the brake circuits. Here, the rate of pressure increase depends on the delivery rate of the pump. In large vehicles the delivery rate of the pump is about 400 Watt, the rate of pressure increase is therefore considerably lower than in the pressure range up to the cut-out point.
  • Should the BKV fail the piston surface area of the Hz causes very high pedal forces. In case of a BKV failure the foot effort required for reaching a certain pressure is thus about five times higher than with the BKV intact. Also, due to the return springs and friction in the Hz and the vacuum brake booster the initial pedal force is very high. From intermediate vehicle categories upward a standard driver is thus practically not able to achieve a reasonable deceleration should the BKV fail. Furthermore, the driver will also be confused by the fact that the initial actuating forces are unusually high.

In systems with travel simulator the Hz can be more freely dimensioned as the pedal travel and the piston travel are not fixedly coupled. In principle, a piston travel increase of approx 20-30% is used and at the same time the entire piston travel is employed for braking at high locking pressures including fading. This enables the use of HZ piston surface areas of only 50% of the conventional design, smaller pedal forces thus being sufficient for generating the required brake pressure. Electromotive brake boosters such as those described in DE 102005018649.19 and DE 102006059840.7 particularly lend themselves for a travel simulator system. In a corresponding embodiment these solutions have the additional advantage of very small friction and restoring forces of the electric motor, causing about 50% lesser response forces. Thus the force/travel characteristic of the pedal during failure of the electromotive BKV compared with the effective brake power assistance is not very strongly different. Despite these advantages, analogous to the dimensioning of the Hz of the conventional brake system initially described, several sizes of master brake cylinders are required to cover the diverse volume intakes from small cars to heavy SUVs. The large piston surface areas of the master brake cylinder for heavy vehicles cause very high pedal forces.

The object of the invention is to provide a brake system that may be employed in several sizes of vehicles wherein in heavy vehicle categories, too, only minor pedal forces need to be applied.

This object is achieved according to the invention with a brake system including the features of claim 1. Further advantageous embodiments are provided through the features of the dependent claims.

The invention is based on the idea of using only one smaller diameter master brake cylinder having a size of master brake cylinder sufficient, for example, for a lower class of vehicles for larger vehicles as well. This is possible particularly with an electromotive brake system as long as the volume of the working chambers of the master brake cylinder is effectively increased by re-feeding. With this, re-feeding always takes place when the pistons of the master brake cylinder have nearly reached their end position during braking and no further build-up of pressure is possible. Prior to re-feeding decoupling of the brake circuits by means of the valves already present takes place. Then the piston drive of the master brake cylinder moves the piston(s) back, while at the same time additional brake fluid is supplied from at least one reservoir into the working chamber or chambers, respectively, of the master brake cylinder. Once the re-feeding process is completed the master brake cylinder can be coupled again to the brake circuits and the pressure in the brake circuits can be increased further. For this each conveying unit is again disconnected or decoupled from the brake circuit by means of the switched valves.

The invention advantageously provides at least one additional conveying unit comprising a reservoir and a valve. As long as the master brake cylinder provides one or two working chambers said one working chamber or both may be charged via a single delivery unit. However, it is advantageous to assign at least one re-feeding conveying unit to each respective working chamber of the master brake in order to advantageously ensure a decoupling of the working chambers.

For heavy vehicles a master brake cylinder with reduced diameter may be used wherein the volume of the master brake cylinder is under dimensioned relative to the volume intake of the wheel brakes. By providing a delivery unit for extreme pressure requirements, the necessary additional volume of brake fluid may be obtained from a reservoir, if required, this subsequently being re-fed into the brake circuits by the electromotively driven BKV.

The requirement for re-feeding is identified using the distance travelled by the master brake cylinder and the built up pressure. If, for example, piston travel is spent at 140 bar then additional brake fluid is delivered from the re-feeding chamber into the Hz so that the pressure may be increased to maximum pressure.

From small cars through to SUVs, it is thus possible to employ the same master brake cylinder. The additional conveying unit must be installed from that vehicle class upward, in which the volume of the master brake cylinder is not sufficient to cover all extreme situations. The dimensions of the master brake cylinder determine from which class onward a conveying unit will be necessary. In case of a failure of the brake power assistance (BKV) high-end vehicle classes benefit from substantially lower pedal forces than in conventional systems due to the small piston surface area of the master brake cylinder. Therefore the pedal feel will not vary as much from the normal situation with the brake power assistance intact, which will unsettle the driver less.

By standardising and reducing the vehicle variations considerable cost reductions for manufacturing and lower logistics costs for procuring and stocking of spare parts are possible. The implementation of the re-feeding component is easy and thus also reliable. Also, the function is diagnosable.

Advantageously, the conveying unit(s) is/are respectively disposed on or integrated in the housing of the electromotively driven brake booster.

The re-feeding chamber may be used for yet another function, namely for adjusting the brake lining clearance. As described in DE (E114), contacting brake linings cause considerable additional fuel consumption. This application describes how this is achieved through the negative pressure in the brake line and brake pistons and through specific piston control and activation of the switching valves. In this application the activation is very complex if the piston cannot be retracted from its initial position, as this means additional constructive efforts regarding the electromotive brake booster. On the other hand, with the re-feeding chamber it is simple as a corresponding volume is temporarily fed from the HZ into this chamber and the piston of the HZ is moved back by switching the valves accordingly to generate a negative pressure in preferably one wheel cylinder. The remaining wheel cylinders are served consecutively. In doing so, with measuring the negative pressure via the pressure gauge and controlling the travel of the push rod piston accordingly, a clearance can be generated in the wheel cylinder. This brake clearance may be eliminated again at any time during or after braking. With an external signal, the brake linings can therefore be brought into contact with the brake disk again prior to the beginning of the braking operation. With this so-called pre-charging it is possible to reduce the amount of brake travel, particularly when the pre-charging pressure has already reached a pressure level of 5 bar.

Below two possible embodiments of the invention are explained in more detail with reference to drawings, in which:

FIG. 1 is a brake system according to the first embodiment including one delivery unit for each working chamber of the master brake cylinder;

FIG. 1 a is a diagram of travel vs. pressure for the brake system in FIG. 1 of two vehicles having different wheel brake volumes;

FIG. 2 is a brake system according to a second embodiment including a coupled delivery unit for both working chambers of the master brake cylinder;

FIG. 3 is a brake system according to a first embodiment including one solenoid valve each between the working chambers of the master brake cylinder and the reservoir.

FIG. 1 shows the basic construction Of an electromotive brake booster as described in DE 102005018649.19, DE 102006059840.7 and DE 102005003648, the entire disclosure content of which hereby being incorporated by reference into the application. When the BKV is inactive the pedal is decoupled from the Hz. The pedal force is measured by the travel simulator, not shown, which generates the familiar pedal feel.

Pedal travel sensor 11 detects the pedal travel, which can be associated with a desired brake pressure by means of a characteristic. Thus the brake booster 2 acting on the push rod piston 3 of the master brake cylinder 5 is activated by actuation of the brake pedal 1. The floating piston 4 is moved by the volume displacement and pressure. Both piston 3 and piston 4 cause the generation of pressure in the respective brake circuits. The corresponding brake fluid is provided in the reservoir. For details of the construction of the known master brake cylinder reference should be made to DE 102005018649.19, DE 102006059840.7 and DE 102005003648. It is known that in travel simulator systems the pedal travel and the piston travel can vary. During braking with a high friction value the piston runs ahead of the pedal. The re-feeding process takes place when the piston 3, 4 reaches the end region of travel. In the process firstly the control valves 7 are closed and the pressure achieved is trapped in the wheel brakes. Subsequently, the re-feeding valves 8 are opened. At the same time the push rod piston 3 is retracted by the electromotive BKV causing the pressure in the master brake cylinder to fall towards zero (0). The stored brake fluid from the already charged re-feeding chambers 20 is delivered into the working chambers A₁, A₂ of the master brake cylinder by means of the spring 10 and the piston 9. Preferably, there is a positive pressure, e.g. 5 bar, in the re-feeding chamber 20 so that the brake fluid is positively delivered into the master brake cylinder. Following this the re-feeding valves 8 are closed and the control valves 7 are opened. Through an appropriate motor drive the brake fluid is now displaced into the brake circuits 22, so that the pressure in the respective brake circuits 22 increases further, depending on the position of the valves. Thus a further rise in pressure is possible without pistons 3 and 4 reaching the end region (left-hand position). Optionally, re-feeding is also possible in one brake circuit only. With an appropriate design of piston surface area and piston travel the missing volume in the Hz can be kept available in the re-feeding chamber to cover all extreme cases. With a corresponding design of the spring the bias of the spring 10 makes for a charge pressure of e.g. 5 to 10 bar. In combination with a re-feeding valve 8 with a large aperture this enables fast re-feeding into the working chambers A₁, A₂ in, for example, about 50 ms, thus preventing a significant delay in the pressure increase.

The re-feeding valves 8 should be optimized for flow and switching times. The valves 8 which should preferably be of a normally closed design may have a large valve seat cross-section. By using a conventional coil the valve 8 can then open for medium high pressures such as for example 50 bar only. This is not disadvantageous for re-feeding as the switching of the re-feeding valves takes place at about 10 bar. There is therefore no need for expensive pressure-compensated valves for re-feeding. For reasons of time it might make sense to not re-feed the entire volume in the re-feeding block 20 in one go. If the piston 3, 4 approaches the end position for example at 140 bar then at first a volume sufficient for a build-up of pressure to 170 bar may be re-fed. If the pressure is to rise further, at 170 bar the remainder of the volume may be re-fed in a new re-feeding step for a maximum pressure of for example 200 bar. Since the first re-feeding step is sufficient for the majority of cases the time delay in the build-up of pressure during re-feeding can be reduced for these braking operations.

The re-feeding chamber 20 may be charged and diagnosed after charging at the assembly line end or during service, at each vehicle start or during the, acceleration phases. For this the maximum pressure in the re-feeding chamber for example 10 bar is preferably introduced at a controlled pressure via the motor drive. When the re-feeding valve 8 is opened at this point the push rod piston 3 must not move. However, should this be the case then this will indicate a leak in the piston seal or a leaking re-feeding valve 8. The differential volume is identifiable by the piston travel S_K. The differential volume and the diagnostic intervals enable detection of the extent of the leakage. For this purpose the master brake cylinder is adjusted to the maximum re-feeding pressure. In addition, it can now be checked for a jamming re-feeding valve 8 or piston 9. As soon as the re-feeding block 20 is charged again the piston 3 is retracted. By looking at the progression of the pressure/volume characteristic it can then be determined if the re-feeding piston 9 is being moved along or if the re-feeding valve 8 switched.

Alternatively, the charging level of the re-feeding chamber 20 can be inspected by closing the control valves 7, adjusting the Hz to the maximum charge pressure of the re-feeding chamber 20, e.g. 10 bar, controlling the position of the piston to be the manipulated variable, opening the re-feeding Valves 8 and monitoring with the pressure sensor 12 for a pressure drop in the Hz.

By adapting the re-feeding volume it is thus possible to use the same basic system for several classes of vehicles. In conventional solutions consisting of Hz and vacuum BKV, individual sizing must be used for each class of vehicle meaning higher costs for the logistics in production and repair.

Moreover, in case of a failure of the brake power assistance the smaller piston diameter generates significantly higher pedal forces.

Since in the travel simulator system the venting condition of the brake system can be regularly checked by means of the pressure/volume characteristic, the total volume of the brake actuation consisting of the volume of the master brake cylinder and the displacement volume of the re-feeding block can be reduced in comparison to conventional systems. There is no longer any need to provide the additional safety volume for a poorly vented volume as is the case in conventional systems.

A further possibility for monitoring the charge condition of the re-feeding chamber 20 is to employ an optional sensor 24. This sensor detects the position of the piston 9. The sensor 24 may be designed as positional resolution sensor or as a switch that detects the position of the piston 9. This sensor may be used for diagnostics or for the defined control of the piston in order to be able to provide a sufficient volume for the function of generating negative pressure.

FIG. 1 a shows the progression of pressure p across piston travel S_N of the push rod piston 3 for a small car A and a sport utility vehicle (SUV) B. Both vehicles are using the same master brake cylinder. The dotted line shows the limits at p_max and at the end of the piston travel. During a braking operation on dry tarmac the small car reaches locking pressure p₁at a piston travel of only 40%. In contrast, in graph B the SUV has a distinctly higher volume intake, i.e. piston travel, so that p_ais reached at for example 70% of piston travel S_N. Mention must again be made of the fact that in both cases due to the use of the travel simulator 2 the maximum pedal travel is limited to for example 40%. The Hz piston runs ahead of the pedal and the real piston travel is not identifiable at pedal 1. With the Hz volume of a small car for example 140 bar may be reached in the SUV. If the pressure in B is further increased, for example during fading, additional volume for the increase in pressure to for example 200 bar is provided on reaching S_N through the previously mentioned re-feeding process N. In FIG. 1a it becomes apparent how piston 3, 4 moves back and is then able to build up the pressure to p_MAX.

During the reduction of the brake pressure at a pressure p₂ or a suitable position bf the piston the additional volume is displaced back into the re feeding block 20. This process is called return feed and is designated R2 in FIG. 1a. During the reduction of pressure for example at 50 bar with opened control valves 20 the re-feeding valves 8 are opened. Thus the re-feeding chambers 20 are quickly charged due to the high differential pressure at re-feeding valve 8. There is therefore no need to interrupt the pressure reduction at the wheel for the return feed process. However, the result of this is en abrupt drop of pressure at the wheel brakes, which may be noticeable for the driver. To avoid this, the pressure reduction gradient or the charging rate of the re-feeding chamber 20 may be varied by means of the pressure sensor 12 and PWM control of the re-feeding valves.

In FIG. 1a R2 shows an alternative return feed process. During the reduction of pressure firstly the control valves 7 are closed and the re-feeding valves 8 are opened. Through the corresponding movement of the Hz piston 3 the re-fed additional volume can now be displaced back into the delivery unit. Subsequently the control valves 7 are opened again and the pressure reduction in the wheel brakes can continue. During this return feed phase no pressure reduction, occurs at the wheel brakes. It is therefore important here, too, that this process takes place quickly so that the driver does not notice any interruption of the pressure reduction.

During return feeding as during re-feeding no reaction at the pedal will be noticeable by the driver. This controlled return feed into the re-feeding chamber 20 enables It to be ensured that the pressure in the initial position of push rod piston 3 and floating piston 4 nearly equals the ambient pressure so that the primary sleeves 23 slide over the inlet openings 21 to the reservoir 6 free from any differential pressure. This fact is necessary to ensure a sufficiently long lifetime of a so-called plunger cylinder as shown in FIG. 1. Thus the use of a more expensive and larger master brake cylinder with a so-called central valve, which must at present be used for brake systems with ESP equipment, can be waived.

FIG. 2 shows an alternative second embodiment of the invention in which volume is also re-fed from a re-feeding chamber into the master brake cylinder.

This unit consists of a cylinder 13 containing two pistons coupled to the drive of the piston 3 via a rod 16 above a tappet. When advancing to build up pressure the drive acts directly on piston 14, when moving back the rod 16 is carried along via the spring 17. This serves a safety purpose so that for example in case of the solenoid valves malfunctioning or the rod 16 jamming the piston 3 can travel back to initial position for a full reduction of pressure.

For re-feeding two circuits are possible. With this, on the right-hand side of the piston solenoid valve 18a and the control valves 7 are closed as the piston 3 is being moved back, while the volume reaches the master brake cylinder 5 via the open solenoid valve 18a through the lip seal. During the subsequent advancing movement solenoid valve 18a is closed again and solenoid valve 17a is opened leading to a further increase in pressure. On the left-hand side of the piston solenoid valve 19 and the control valves 7 are closed during retraction of piston 3 and 14, respectively, and solenoid valve 18 is opened to draw further volume from the reservoir 6. During the advancing movement piston 14 delivers the volume into the Hz via the open solenoid valves 19 and 7 for a further increase in pressure.

In the solution according to FIG. 1 re-feeding is restricted to the volume of the re-feeding chamber 20. In contrast, in the solution according to FIG. 2 re-feeding can continue until the reservoir is empty.

FIG. 3 shows the possible use of the delivery unit 20 in order to positively adjust a lining clearance at the wheel brake RB. The construction of a wheel brake RB is generally well known and will not be explained in greater detail here. For further information see Bremsenhandbuch, 2^nd edition, Vieweg 2004.

To adjust a lining clearance between the brake disk and the brake lining a negative pressure is temporarily generated in the tandem master brake cylinder (THZ) 3, 4, 5. The brake pistons in the wheel brakes are therefore positively retracted leading to a gap between the wheel lining and the brake disk. This results in the elimination of any residual frictional effect between brake linings and brake disk. The negative pressure can be generated using the re-feeding chamber 20, the basic function of which has already been explained in FIG. 1. As an enhancement of the construction in FIG. 1, in FIG. 3 a switching valve 18 each has been placed between the working chambers A1 end A2 of the master brake cylinder and the reservoir 6.

During normal operation the re-feeding chambers 20 are not fully charged. They contain a sufficient volume to provide brake fluid for high pressure requirements as in the case described in FIG. 1, but can also take in additional volume.

At the start of the adjustment of the lining clearance piston 3 is advanced by the motor drive 2. Piston 4 moves correspondingly along with it. With re-feeding valves 8 opened, the brake fluid is thus displaced into the re-feeding chamber 20 that is only partly charged. Subsequently, the re-feeding valves 8 are closed. At this point solenoid valves 17 will be closed and one of the control valves 7 will be opened. The piston 3, still in its extended position, is retracted partly towards its initial position by the motor spindle drive. This results in a negative pressure which transfers via brake line 22 to the respective wheel brake RB, the control valve 7 of which is open. At this point the remaining 3 wheel brakes are retracted by sequentially opening the respective control valves. Through the surface-area-to-brake-piston ratio the travel of piston 3 is proportional to the travel of the brake piston. In this phase the negative pressure is evaluated so that the movement of the piston is only evaluated under pressure level or a temporal progression of pressure. Temporal progression of pressure means that when the negative pressure through friction of the piston is constant then this equally corresponds to movement of the brake piston. Finally the solenoid valves 18 are opened again. Thus the negative pressure in the THZ 5 is eliminated. It is the task of the solenoid valves 18 to prevent brake fluid reaching the working chambers A1 and A2 of the THZ from the reservoir via the THZ seals during the negative pressure phase in the THZ. It is also possible to retract all brake pistons of the wheel brakes RB simultaneously by opening all of the control valves 7 in the negative pressure phase.

As initially mentioned, during normal operation the re-feeding chambers are not fully charged so that they can take in volume for the adjustment of the lining clearance. The charging condition may be monitored via sensor 24. Alternatively, it is also possible to initially fully charge the re-feeding chambers and to open the re-feeding valves 8 temporarily with piston 3 retracted, control valves 7 closed and solenoid valves 18 opened in order to let a defined volume escape from the re-feeding chamber. A further possibility is to completely empty the re-feeding chambers and to introduce a defined volume via the piston travel 3. Here it is advantageous if both re-feeding chambers 20 are charged separately from each other so that one chamber 20 is always fully charged and the volume for an emergency as described in FIG. 1 is provided.

Due to the adjusted lining clearance there is an increased distance between the brake lining and the brake disk. This would interfere with the braking operation as it causes additional volume intake and thus a loss of travel of the piston 3. It is therefore important that prior to a possible braking operation the brake linings are brought back into contact with brake disk. This is called a pre-charge.

For this the brake fluid from re-feeding chambers 20 may be used. At first solenoid valves 12 are closed, control valves 7 are opened and subsequently re-feeding valves 8 are opened. Thus the springs 10 displace the brake fluid from the re-feeding chambers 20 via the piston 9 into the wheel brakes RB. The required volume can be controlled through the position of piston 9 supplied by sensor 24. Alternatively the pre-charge volume may be adjusted from the opening time of the re-feeding valves and the charging pressure of the re-feeding chamber 20. The pressure sensor 12 also allows detection of when the lining clearance has been eliminated. As soon as the brake linings are in with the brake disk the pressure in the brake circuit rises. Concerning the reduction of the brake travel, a pre-charge to about 5 bar is even more effective, this requires an external sensor, e.g. a pedal proximity sensor.

One method which is applicable when, with the lining clearance adjusted, the re-feeding chambers 20 have become discharged for example due to a leak, provides the following working steps: The re-feeding valves 8 remain closed at first, the control valves opened. Piston 3 is actuated by the motor drive so that a corresponding volume of brake fluid is delivered into the brake circuits until the linings are brought into contact. Subsequently control valves 7 are closed and piston 3 is retracted again. This generates negative pressure in the working chambers A1 and A2. As soon as piston 3 has reached its initial position the negative pressure draws the corresponding differential volume from the reservoir.

LIST OF REFERENCE NUMBERS

• 1 Brake pedal
• 2 Motor drive with travel simulator
• 3 Push rod piston DK
• 4 Floating piston
• 5 Master brake-cylinder Hz
• 6 Reservoir
• 7 Control valves
• 8 Re-feeding valve
• 9 Piston
• 10 Spring
• 11 Pedal travel sensor
• 12 Pressure sensor
• 13 Cylinder
• 14 Double piston
• 15 Tappet
• 16 Spring
• 17 Solenoid valve
• 18 a Solenoid valve
• 19 Solenoid valve
• 19 a Solenoid valve
• 20 Re-feeding chamber
• 21 Inlet openings
• 22 Brake circuit
• 23 Primary sleeve
• 24 Sensor
• p_max Maximum brake pressure
• p₁ Locking pressure for μ=1.0 (dry tarmac)
• A₁, A₂ Working chambers of the HZ
• A Pressure/travel characteristic for small car
• B Pressure/travel characteristic for SUV
• F Conveying unit
• L Lines
• N Re-feeding
• R1, R2 Return feed
• S_K Piston travel of the HZ
• S_N Re-feeding position
• p₂ Charge of re-feeding chamber
• BL Brake line
• ZL Supply line

Claims

1.-33. (canceled)

34. A brake system including an electromotive drive device for a brake booster having at least one piston, the at least one piston being driven by the drive device for pressure build-up and pressure relief in at least one wheel brake and being mechanically adjustable by means of a brake actuating device, particularly in the form of a brake pedal, only in the event of malfunction of the drive device, at least one decoupling valve for closing and opening the brake line being arranged in the brake line between every wheel brake and a working chamber of the brake booster, wherein at least one conveying unit is provided which delivers, when required, brake fluid from a reservoir to the respective working chamber of the brake booster via at least one supply line that is connected to a brake line or the brake booster, respectively, especially to a working chamber, a controlled valve for opening and closing the respective supply line being arranged in each supply line.

35. The brake system according to claim 34, wherein the conveying unit is a piston cylinder system, the at least one piston of which being pressurised by a drive or spring and the drive or spring, respectively, generating the delivery pressure, wherein a reservoir is formed by a working chamber of the piston cylinder system.

36. The brake system according to claim 34, wherein a conveying unit is associated with each working chamber.

37. The brake system according to claim 35, wherein the working chamber of the piston cylinder system of the conveying unit is chargeable by means of the brake booster.

38. The brake system according to claim 34, wherein the conveying unit is a piston cylinder system, the at least one piston of which being driven by the drive of the brake booster or being coupled to its piston.

39. The brake system according to claim 38, wherein the at least one piston of the piston cylinder system is coupled to the driven piston of the brake booster via a tappet.

40. The brake system according to claim 39, wherein during pressure build up in the brake booster, the tappet carries along the at least one piston of the piston cylinder system of the conveying unit by means of a positive locking fit, while the tappet carries along the piston(s) of the conveying unit during pressure relief in the brake booster only via a spring element.

41. The brake system according to claim 38, wherein each working chamber of the piston cylinder system communicates with the reservoir of the brake system via a line, a controlled shut-off valve for optional interruption of the line being disposed in the line.

42. The brake system according to claim 34, wherein the switching valve in the supply line is adapted for low pressure and has a passage of large cross-section.

43. The brake system according to claim 34, wherein the position of the piston of a re-feeding chamber is determined by a sensor.

44. A method for operating a brake system according to claim 34, comprising: during re-feeding of brake fluid to a working chamber of the brake booster, decoupling the wheel brakes connected to the working chamber from the working chamber using the decoupling valves in order to maintain the pressure; andat the same time the valve in the supply line is open, retracting the piston or pistons of the brake booster to enlarge the working chamber or chambers thereof.

45. A method according to claim 44, wherein the brake fluid is re-fed several times by means of one or more delivery units in order to reduce the time delay during build up of the brake pressure.

46. A method according to claim 44, wherein recharging the reservoir of the delivery unit the wheel brakes connected to the respective working chamber are decoupled from the working chamber using the decoupling valves and brake fluid is delivered from the working chamber of the brake booster to the reservoir by pressure build up in the brake booster, the valve in the supply line being moved to its closed position after the charging operation.

47. A method according to claim 44, wherein during recharging of the working chamber of a piston cylinder system driven by the drive of the brake booster the valve disposed in the line that connects the working chamber with the reservoir of the brake system is opened and that the valve disposed in the supply line that connects the working chamber of the conveying unit with the brake booster is closed.

48. A method according to claim 44, wherein the gradient of pressure relief in the re-feeding chamber is controlled by PWM operation of the re-feeding valves via the pressure sensor.

49. A method for operating a brake system according to claim 34, comprising: closing the valves to generate a negative pressure in the working chambers of the brake booster; andretracting the pistons of the brake booster in order to enlarge the working chambers so as to particularly adjust a lining clearance in the wheel brakes with simultaneously opened valves.

50. A method for operating a brake system according to claim 34, comprising: displacing part of the volume in the master brake cylinder into one or more re-feeding chambers by means of the master brake cylinder when the re-feeding valve is opened; andsubsequently, lifting the brake pistons in the wheel brakes off the brake discs with the re-feeding valve closed and the control valve opened by retracting the piston of the master brake cylinder in order to achieve a lining clearance.

51. A method according to claim 49, wherein the lining clearance in the wheel brakes is adjusted consecutively or simultaneously or in pairs.

52. A method according to claim 49, wherein during the generation of a negative pressure by means of retracting the pistons of the master brake cylinder a valve disposed in the line connecting the reservoir with the master brake cylinder is closed.

53. A method according to claim 49, wherein a defined stroke of the piston is carried out on the basis of the pressure in the brake line or the master brake cylinder determined by means of a sensor.

54. A method according to claim 49, wherein the piston of the re-feeding chamber is in an intermediate position, in order for this to take in further volume for negative pressure control or adjustment of the lining clearance.

55. A method according to claim 54, wherein this intermediate position is achieved by specific opening times of the solenoid valves and returning of the volume to the working chambers of the master brake cylinder during those times in which no braking occurs.

56. A method according to claim 49, wherein the adjustment to the intermediate position of the piston of a re-feeding chamber is achieved by means of the position sensor.

57. A method according to claim 49, wherein the brake fluid volume stored in a re-feeding chamber is provided via an external signal for pre-charging the brake by opening the re-feeding valve and optionally one or all of the control valves prior to the start of a braking operation.

58. A method according to claim 57, wherein said volume during control of the negative pressure is sized so as to generate a pressure of about 5 bar in pre-charging the brake.

59. A method according to claim 49, wherein adjusting a brake clearance between brake disc and brake lining of a wheel brake by means of the pistons a negative pressure is generated in the working chambers of the brake booster, at the same time the respective valve or valves being opened and the line that connects the respective working chamber of the brake booster with the reservoir or the re-feeding unit being closed by the valve.

60. A method according to claim 49, wherein eliminating the lining clearance the valves are closed and subsequently the valves as well as the respective valve are opened.

61. A method according to claim 60, wherein at a pressure resulting in a braking force in the respective brake line the brake line is connected with the reservoir by opening the corresponding valves.

62. A method according to claim 49, wherein the valves are closed and the valve or valves are opened to eliminate the brake clearance, hydraulic medium subsequently being delivered to the wheel brake by means of the pistons via the brake line.