GB2309275A

GB2309275A - Providing "feel" at the brake pedal

Info

Publication number: GB2309275A
Application number: GB9705560A
Authority: GB
Inventors: Matthias Hartl; Harry Troster; Rainer Freitag; Martina Richerzhagen
Original assignee: Daimler Benz AG; Mercedes Benz AG
Current assignee: Daimler Benz AG
Priority date: 1995-11-23
Filing date: 1996-11-22
Publication date: 1997-07-23
Anticipated expiration: 2016-11-22
Also published as: GB2309275B; GB9705560D0

Description

Brake linkage and brake master cylinder The invention relates to a brake linkage and brake master cylinder, such as are generally known in motor vehicles.

In the generally conventional design of brake linkages and brake master cylinders, there is an at least virtually play-free connection between the brake pedal and a piston of the brake master cylinder. There is a fixed relationship between the movement of the piston of the brake master cylinder and the pressure of the brake fluid to the main brake lines. This ensures that the driver can always generate a brake pressure in the main brake lines by actuating the brake pedal.

Furthermore, it is known, for example from DE 43 35 769 C1, to interrupt the main brake lines between a brake master cylinder and wheel-brake cylinders fluidically by means of change-over valves. The brake pressure in the wheel-brake cylinders which is necessary for actuating the wheel brakes is generated, here, by means of a separate power source, in particular by means of plungers, and is controlled or regulated by a brake-pressure control device. As a result of the closing of the change-over valve, no brake fluid can escape from the brake master cylinder into the wheel-brake cylinders, so that, without any appreciable pedal travel, high pressure in the brake master cylinder immediately opposes the actuation of the brake pedal. The brake pedal does not allow any pedal travel and feels "hard". The driver may therefore proportion the actuation of the brakes inaccurately.

The present invention seeks to develop further a brake system operated in this way, to the effect that, when the brake-pressure control device is functioning normally, the brake pedal can execute a pedal travel, even with the change-over valves closed, and, on the other hand, in the event of faults in the functioning of the brake-pressure control device, brake pressure can be generated in the conventional way in the brake master cylinder and the wheelbrake cylinders by actuating the brake pedal, the brake pedal having, at most, an insignificant idle travel.

According to one aspect of the invention there is provided a brake master cylinder with a plunger piston loaded by a brake linkage, and with a floating piston which is arranged between two working spaces and which separates the working spaces fluidically from one another, each of the two working chambers opening into a main brake circuit, wherein the first working chamber located between the floating piston and the plunger piston has a displacement duct leading out of the brake master cylinder, the displacement duct being capable of being closed by means of a switching valve.

According to another aspect of the present invention there is provided a brake linkage adapted to be connected, on the one hand, to a brake pedal and, on the other hand, to a piston of a brake master cylinder, brake pressure being generated in at least one working space as a result of movement of the piston in the brake master cylinder, wherein the brake linkage comprises two rods adapted to be relatively movable in relation to one another within a predetermined range of movement, a switchable blocking device for selectively blocking the relative movement between the two rods being provided.

According to a first embodiment of the invention, between two portions of the brake linkage there is a predetermined movement play for relative movement between the part elements of the brake linkage. The brake linkage consists of two rods arranged coaxially relative to one another, one of the rods having a hollow-cylindrical end, in which one end of the other rod is guided. The relative movement between the two rods can be blocked by means of a blocking device. Thus, when the power-operated brake system is functioning, a pedal-travel simulation is carried out, so that the pedal does not feel hard to the driver, whereas, when a fault occurs in the power-operated brake system, brake pressure is generated in the brake master cylinder as a result of the actuation of the brake pedal.

According to further embodiments of the invention, switching into the blocking position of the blocking device takes place by means of a positive control, in particular by means of a switching boss which acts on a blocking pawl, the blocking pawl being arranged on one of the rods and, as a result of actuation, engaging into a detent nose which is formed on the other rod.

According to another embodiment of the invention, the actuation of the blocking device takes place by means of a control signal which is generated by a brake-pressure control device.

According to a further embodiment of the invention, a displacement duct is made in a brake master cylinder having two working chambers, the first working chamber being delimited, on the one hand, by a plunger piston loaded by means of the brake linkage and, on the other hand, by a floating piston separating off the second working chamber, the said displacement duct leading from the first working chamber out of the brake master cylinder. The displacement duct can be closed by means of a switching valve.

According to one embodiment of the invention, the switching valve is arranged in the first working chamber of the brake master cylinder. According to further embodiments, the switching valve is closed by virtue of a movement of the floating piston, insofar as movement beyond a specific degree results in a reduction in the volume of the second working chamber of the brake booster.

According to further embodiments, it is advantageous if the displacement duct opens into a pressure accumulator.

The invention will now be explained below with reference to the exemplary embodiments illustrated in the drawings, in which: Figures 1 to 3 s h o w d i f f e r e n t v a r i a n t s of a brake linkage according to the invention with positively controlled and with controlled actuation of the blocking device, which are based on the same arrangement in respect of the brake master cylinder, Figure 4 shows a further embodiment of a brake linkage with a blocking device which switches by positive control, Figure 5 shows an embodiment of a brake booster, and Figure 6 shows a second embodiment of a brake booster.

Figures 1 to 3 show three variants of the design of brake linkages according to the invention. The designs of the brake master cylinder 9, main brake lines 12, 13, change-over valves 14 and control device 15 correspond to one another. As long as the brake pedal is not actuated, the change-over valves 14 are in their non-actuated position of rest, and the fluidic connection between the two working chambers 16, 17 of the brake master cylinder 9 and the respectively associated wheel-brake cylinders, which are not shown here, is made via the main brake lines 12, 13.

When the vehicle brake is actuated, then, as long as there is no fault warning relating to the functioning of the power-operated brake system (also referred to below as the normal situation), the change-over valves 14 are closed by means of the control unit 15. Consequently, no brake fluid can escape from the working chambers 16, 17 of the brake master cylinder 9 into the main brake lines 12, 13 and the wheel-brake cylinders. The necessary wheel-brake pressure corresponding to the driver's wishes is generated in the wheel-brake cylinders by the power-operated brake device, if appropriate taking account of brakepressure regulations which are intended to stabilize the vehicle.

By means of the actuating force exerted by the driver and acting on the brake pedal, the pistons 7 and 11 of the brake master cylinder 9 are first shifted in the direction of a pressure build-up in the working chambers 16, 17, until the snifting holes 18, 19 leading to a brake-fluid reservoir and belonging to the working chambers 16, 17 are closed. Without a build-up of brake pressure in the working chambers 16 and 17, the pistons 7, 8 can cover only this short distance.

However, if it has been found that there is a fault in the functioning of the power-operated brake system, the closing of the change-over valves 14 by means of the control unit 15 is discontinued. As soon as a relative movement of the two rods in relation to one another is blocked by the respective blocking device which is described in more detail below, the actuating force of the brake pedal, exerted by the driver and, if appropriate, boosted by a brake booster (not shown), is exerted on the pistons 7, 11 of the brake booster via the mechanical connection. In the conventional functioning of the brake master cylinder, the pistons 7, 11 move as a result of the force introduced and reduce the volume of the working chambers of the brake master cylinder. The snifting holes 18, 19 first close, and subsequently brake fluid is displaced out of the working chambers into the wheel-brake cylinders. The brake pressure counteracting the actuation of the brake pedal is built up in the working chambers and the wheel brake cylinders, until an equilibrium of forces is established. The movement of the pistons, in particular the movement of the piston 7, brings about a travel of the brake linkage 1, 3; 21, 23; 31, 33 which is in a fixed ratio to the actuating force of the brake pedal. Furthermore, the brake master cylinder 9 shown has all the fall-back functions provided in generally conventional brake master cylinders, in the event of leakages in individual brake circuits, these functions not being described in any more detail here.

The previous accounts apply equally to Figures 1 to 3 and to Figure 4, the brake master cylinder 49 being shown only diagrammatically in Figure 4.

Its design can correspond to that of the brake master cylinder of Figures 1 to 3.

According to the design illustrated in Figure 1, part of the brake linkage 1, 3 is guided in a housing 8, in which the relative movement between the two rods 1, 3 takes place. The housing 8 is connected to the housing of the brake master cylinder 9. The brake linkage rod 1 connected mechanically to the brake pedal projects with its hollow-cylindrical end 120 into the housing 8. One end 123 of the second rod 3 is guided in the hollow-cylindrical end 120, the other end of the second rod being connected to the plunger piston 7 of the brake master cylinder 9. The movement play in the relative movement of the two rods in relation to one another is determined by the distance over which the end 123 of the second rod 3 can penetrate into the hollow-cylindrical end 120 of the rod 1. Formed on the hollow-cylindrical end 120 of the first rod 1 is a supporting ring 121, and a supporting ring 122 is likewise formed, outside the movement play, on the second rod 3 on the side facing the first rod 1. A return spring 2 is arranged between the two supporting rings. A blocking pawl 4 is articulated on the supporting ring 122 of the second rod 3, there being arranged in the joint 5 a spring, in particular a torsion-stressed helical spring, which holds the blocking pawl in the position releasing the relative movement. A switching boss 6 projecting inwards and located approximately level with the joint 5 is arranged or shaped out on the housing 8. The switching boss is preferably formed from slightly elastic material. The switching boss projects beyond the length of the return spring 2 in the position of rest into the region of the first rod 1, on which a detent nose 125 is formed. Opposite the detent nose 125, at least one preferably tooth-shaped detent hole 124, preferably a plurality of these, is formed on the blocking pawl 4.

In the normal situation, the blocking pawl 4 at most bears on the switching boss 6, and the blocking pawl 4 is not actuated. The relative movement between the two rods 1, 3 is possible, within the scope of the movement play, counter to the effect of the return spring 2, and, because the plunger piston 7 does not move beyond the snifting hole 18, the force resulting from the compression of the return spring 2 is felt by the driver as a force which acts counter to him.

In the event of a fault in functioning, the plunger piston moves beyond the position covering the snifting hole 18 and the blocking pawl 4 is pressed down by the switching boss 6. The detent nose 125 now thereby projects into the detent hole 124 and a connection between the two rods 1, 3, which is non-positive in the direction of axial extension of the rods 1, 3, is consequently made. The force acting on the brake pedal is transmitted completely to the plunger piston 7 by this connection. A lengthening of the pedal travel is prevented on account of the rigid connection. After the brake pedal has been released, the switching boss 6 no longer acts on the blocking pawl 4, and the blocking pawl 4 is loaded by the spring in the joint 5 in such a way that the blocking pawl 4 is once again out of engagement with the detent nose 4.

To improve the non-positive connection between the two rods 1, 3, a plurality of, in particular three blocking pawls, with associated detent noses and switching bosses, can be arranged in a radially uniformly distributed manner. The switching bosses can also be designed as a continuous ring in the housing 8.

Figure 2 shows an arrangement which is simplified in relation to Figure 1. The functioning of the design function illustrated in Figure 2 corresponds to the design of Figure 1, so that only the differences in the design are shown here.

The function of the large-size blocking pawl 4 of Figure 1 is performed by the short, approximately triangular blocking pawl 24 which, articulated at a vertex in the joint 25, is held by a spring in the joint 25 in the opening position shown and is pressed down by the switching boss 26 when there is a fault in functioning. In the region of the blocking pawl 24, the second rod 21 has a notch 28, into which the blocking pawl 24 is pressed down. In order to ensure better force transmission, a switching ring 27 is formed on the second rod 21 at that end of the notch 28 which is located on the brake-pedal side. The first rod 23 having the hollow-cylindrical portion 210 is connected to the plunger piston 7. A return spring 22 is arranged inside the hollow-cylindrical portion 210 of the rod 23. The end 29 of the second rod 21 is guided in the hollow-cylindrical portion 210 and bears on the return spring 22.

By means of the blocking device designed in this way, a relative movement between the two rods 21, 23 is blocked only in the direction of actuation of the brake pedal, but not in the direction of release of the brake pedal. The block produced by the blocking device can therefore only be pressure-stressed, but not tension-stressed.

The embodiment according to Figure 3 differs from the two preceding embodiments essentially in that the mechanical connection between the two rods 31, 33 of the brake linkage is made and broken by means of a blocking device which can be activated by means of an electromagnet. The design of the second rod 33 connected to the plunger piston 7 corresponds to the second rod 3 of Figure 1 and likewise has a supporting ring 38 for a return spring 32. In this case, the return spring 32 is once again arranged outside the hollow-cylindrical end 39 between the two rods 31, 33. At the same time, that end of the return spring 32 which faces away from the supporting ring 38 is supported on a sleeve 35. Arranged in the sleeve 35 is an electromagnet 36 which is activated by the control unit 15 via the control line 310. Electromagnet 36 acts on a bolt 34 which is prestressed in the direction of the second rod 33 by the spring 37. The bolt passes through the first rod 31 in a bore 311 and, when the brake pedal is in the non-actuated position, is located opposite a bore or a groove 312 in the second rod 33.

In the normal situation, when brake actuation commences, current is supplied to the electromagnet 36, so that the bolt passes, counter to the effect of the spring 37, into a position in which it connects the first rod 31 and the sleeve 35 mechanically to one another. When the brake pedal is actuated, first the plunger piston is moved in the direction of a reduction in the volume of the working chamber 16 until the snifting hole 18 is closed. subsequently, as a result of the movement of the sleeve 35, the return spring 32 is moved in the direction of the now stationary second rod 33.

If there is a fault in functioning, the activation of the electromagnet 36 is discontinued, so that the bolt 34 passes through the bore 311 and the groove 312. The bolt 34 connects the two rods 31 and 33 mechanically, so that, when the brake is actuated, the forces introduced into the brake pedal act completely on the plunger piston 7. It goes without saying that this design too has the arrangement of a plurality of blocking devices which take the form described and which, arranged radially at a uniform distance from one another, and which improve the transmission of force between the two rods 31, 33 and also between the first rod 31 and the sleeve 35.

Consequently, in this embodiment of the invention, when the switchable blocking device is in the non-actuated position the relative movement between the two rods 33, 31 is blocked, whereas, when the switchable blocking device is in the actuated position shown in the drawing, the relative movement is released.

Figure 4 shows a further embodiment of the invention which causes a blocking of the relative movement between the two rods by means of mechanical blocking pawls. The arrangement differs from the designs according to Figures 1 to 3 in that the return spring 42 is not supported between the two rods 41, 43 of the brake linkage. Instead, the return spring 42 is supported between a sleeve 410 and a supporting wall 411. In this case, the sleeve 410 is mounted displaceably on the rod 41 connected to the brake pedal. The second rod 43 is guided in the hollow-cylindrical end 413 of the first rod 43 and, at the other end, is connected to the piston of a brake master cylinder 49 shown only symbolically. The said rod 43 has a frustoconical bevel 414 at its end projecting into the hollow-cylindrical end 413 of the first rod 41. When the brake linkage is in the position of rest, blocking pawls 44, which are arranged in bores 415 uniformly spaced radially from one another and passing through the wall of the hollow-cylindrical end 413, bear on the frustoconical bevel 414 of the second rod 43. The blocking pawls 44 are loaded in the direction of the frustoconical bevel 414 by means of leaf springs 48 arranged in the wall of the hollowcylindrical end of the first rod 41 and projecting into the bores 415. The sleeve 410 has a groove 46 in the region which is located opposite the blocking pawls 44 when the brake pedal is in the non-actuated position. That end of the groove 46 which is located on the same side as the brake master cylinder 49 is offset forwards in the direction of the brake master cylinder 49 from the corresponding edge of the blocking pawl 44, at most by an amount corresponding to the travel of the piston until the snifting hole of the working chamber is closed. That region of the sleeve which extends in front of the groove in the direction of the brake master cylinder 49 is provided with a sliding rail 45.

In the normal situation, when the brake is actuated the second rod 43 moves until the snifting holes in the brake master cylinder 49 are covered. The first rod 41 slides with its hollow-cylindrical end 413 over the second rod 43, and the blocking pawls 44 are pressed outwards into the groove 46. A mechanical connection between the first rod 41 and the sleeve 410 is made. The relative movement between the first rod 41 and second rod 43 is released, so that the forces acting on the brake pedal are supported via the spring 42 on the supporting wall 411, the spring 42 being compressed.

If there is a fault in functioning and conventional brake operation is taking place, the rod 43 moves further than only to cover the snifting holes.

During the further pressure build-up in the brake master cylinder, the blocking pawls 44 are loaded with a force which presses them outwards. It is no longer possible for the blocking pawls 44 to escape into the groove 46, since the latter has already been overrun. Instead, the blocking pawls 44 are supported on the sliding rail 45. Since they are therefore still located in the space in which the second rod 43 moves relative to the first rod 41, relative movement is blocked and the force introduced into the first rod 41 is transmitted completely to the wheel brakes. An embodiment with an electromechanical, hydraulic or pneumatic triggering of the blocking pawls 44 is also possible.

Figures 5 and 6 show two versions of a brake master cylinder according to the invention which scarcely differ from one another in their design. Each of the two figures shows a sectional representation of the brake master cylinder, only the region of the first working chamber being illustrated in each case. Otherwise, the brake master cylinder can, for example, be designed identically to the brake master cylinder 9 shown in Figure 1.

The design and functioning which are common to Figure 5 and to Figure 6 are described first, and then the particular properties of the two embodiments are shown.

A first working chamber 51 of the brake master cylinder is delimited by the floating piston 52 and the plunger piston 53 in the housing 54 of the brake master cylinder 50. The brake force exerted on the brake pedal is introduced into the plunger piston 53 via the brake linkage 55. When the brake pedal is in the non-actuated position of rest, the working chamber 51 is fluidically connected via the snifting hole 56 to the connection piece 57 and thereby to a brake-fluid reservoir not shown in any more detail. The return spring 58 arranged between the two pistons 52, 53 ensures a specific position of rest of these. The first working chamber has two more fluidic connections which lead out of the brake master cylinder 50. These are, on the one hand, the main brake line 59 and, on the other hand, displacement duct 512. The main brake line 59 can be shut off by means of the change-over valve 510 which can be activated by the control device 511. The displacement duct can be shut off by means of a switching valve described for the first time below. The displacement duct 512 is connected fluidically to a pressure accumulator 513.

When the brake is actuated in the normal situation, first the plunger piston 53 and the floating piston 52 move until the snifting hole 56 of the first 7 working chamber and the snifting hole, not shown in any more detail, of the second working chamber are closed by the respective piston. The main brake line 59 is closed by means of the change-over valve 510. The same thing occurs on the second working chamber not shown in any more detail. In this state, brake fluid can escape from the brake master cylinder 50 only via the displacement duct 512. It passes into the pressure accumulator 513. The actuating force exerted by the driver on the brake pedal is counteracted by the rising pressure in the pressure accumulator 513. The plunger piston 55 can move further in the direction of a reduction in the volume of the first working chamber 51. It is possible for the at least approximately pressure-resistant brake linkage 55 to execute a pedal travel. There is a fixed relationship between the pedal travel and the actuating force of the brake pedal necessary for achieving this pedal travel. With a suitable storage-volume/pressure characteristic of the pressure accumulator 513, it is possible at least approximately to generate the same travel/force characteristic which is obtained when the brake fluid has been displaced out of the brake master cylinder into the wheel-brake cylinders. A brake pressure is fed into the wheel-brake cylinders via the power-operated brake device.

However, if there is a fault in the functioning of the power-operated brake device, the change-over valves 510 are not closed. After the snifting holes 56 are closed, brake fluid can be displaced out of the two working chambers of the brake master cylinder into the wheel-brake cylinders. At the latest shortly after the respective snifting hole 56 has been closed by the plunger piston 53 or the floating piston 52, the switching valve for the displacement duct 512 must also be closed. Brake fluid can escape from the first working chamber only through the main brake line 59, brake pressure being built up in the wheel brakes in the conventional way.

The following descriptions explain the specific properties of the embodiments according to Figure 5 and Figure 6.

According to the design shown in Figure 5, the switching valve of the displacement duct 512 is designed as a mechanically switching valve 520.

Arranged in a valve housing 521, which surrounds the valve chamber 526, is a closing spring 522 which loads a valve tappet 523 passing through the valve housing 521, in such a way that, when the valve tappet 523 is not loaded, a sealing ring 524 formed on the valve tappet 523 closes the only fluidic connection 525 between the first working chamber 51 and the valve chamber 526 in a fluid-tight manner. The displacement duct 512 opens into the valve chamber 526. When the brake pedal is in the non-actuated position, the floating piston 52 pushes the valve tappet 523 so far counter to the effect of the closing spring 522 that a fluidic connection is made between the valve chamber 526 and the first working chamber 51. Consequently, brake fluid can escape from the first working chamber into the pressure accumulator 513 via the displacement duct 512. When the brake pedal is actuated, the floating piston 52 moves away from the valve 520.

In the normal situation, this takes place until the snifting hole of the second working chamber is closed by the floating piston 52. The floating piston 52 subsequently stops, since brake fluid can no longer escape from the second working chamber because the main brake line is shut off by a change-over valve. In this position too, the floating piston 52 presses onto the valve tappet 523 in such a way that the fluidic connection 525 between the first working chamber 51 and valve chamber 526 is made.

If a fault in functioning occurs, the change-over valve of the second working chamber is not closed, so that the floating piston moves further away from the valve 520. At a slight distance (less than 1 mm) after the movement distance which it covered after closing the snifting hole, the floating piston 52 is released from the valve tappet 523 and the sealing ring 524 closes the fluidic connection 525 between the valve chamber 526 and first working chamber 51.

As a result, brake fluid can no longer escape out of the first working chamber 51 through the displacement duct 512. There is therefore a build-up of brake pressure, as in the conventional functioning of a brake master cylinder. The brake pressure is introduced into the wheel brakes via the main brake line 59.

According to the embodiment shown in Figure 6, the switching valve of the displacement duct 512 is designed as an electromagnetic valve 61 capable of being switched by means of the control device 511. In the non-actuated position, the electromagnetic valve closes the fluidic connection between the displacement duct 512 and the pressure accumulator 513. As soon as the brake is actuated, which can be recorded, for example, via the position of the stop light switch, a check is made as to whether there is a fault in the functioning of the power-operated brake system or of the change-over valves. If this is so, the valves 61 and the change-over valves 510 remain non-actuated and the build-up of brake pressure in the wheel-brake cylinders takes place in a conventional way via the brake master cylinder. Otherwise, the change-over valves 510 and the valve 61 are actuated. The fluidic connection between the pressure accumulator 513 and the first working chamber 51 is made. Brake fluid can thus escape from the working chamber in the desired way and, via the plunger piston 53 which continues to be displaceable, it becomes possible for the brake pedal to execute a pedal travel.

According to a design having particularly high operating reliability, the change-over valve 510 and the valve 61 can be combined to form a 4/2-way valve, in one switching position the fluidic connection between the first working chamber 51 and wheel-brake cylinders being opened and the fluidic connection between the working chamber 51 and pressure accumulator 513 being closed, whereas, in the second switching position, the fluidic connection between the first working chamber 51 and wheel-brake cylinders is closed and the fluidic connection between the working chamber 51 and pressure accumulator 513 is opened.

Claims

1. A brake master cylinder with a plunger piston loaded by a brake linkage, and with a floating piston which is arranged between two working spaces and which separates the working spaces fluidically from one another, each of the two working chambers opening into a main brake circuit, wherein the first working chamber located between the floating piston and the plunger piston has a displacement duct leading out of the brake master cylinder, the displacement duct being capable of being closed by means of a switching valve.

2. A brake master cylinder according to Claim 1, wherein the switching valve of the displacement duct is switchable by means of a control device.

3. A brake master cylinder according to Claim 2, wherein the switching valve is closed when the brake is in the non-actuated state, and, when the brake is actuated, the switching valve is transferred into a switching state opening the displacement duct, as soon as the control device generates a signal which represents an undisturbed operation of a power-operated brake system.

4. A brake master cylinder according to Claim 1, 2 or 3, wherein the displacement duct is arranged outside that range of movement of the plunger piston and of the floating piston which is possibly utilized when the brake is actuated.

5. A brake master cylinder according to any one of Claims 1 to 4, wherein the switching valve is arranged in the first working chamber and is fastened to a side wall of the brake master cylinder.

6. A brake master cylinder according to Claim 5, wherein switching operations of the switching valve are generated as a result of a movement of the floating piston in the brake master cylinder, the switching valve being held open when the floating piston is in the non-actuated position of rest, and a closing of the switching valve taking place when the floating piston has moved beyond a predetermined degree in the direction bringing about a reduction in the volume of the second working chamber.

7. A brake master cylinder according to Claim 6, wherein the predetermined degree is slightly greater than the distance which the floating piston must cover in order to cover a snifting hole, which knsures the supply of brake fluid from a reservoir into the working chamber, and thus to break the fluidic connection.

8. A brake master cylinder according to any one of Claims 1 to 7, wherein the displacement duct opens into a pressure accumulator.

9. A brake linkage adapted to be connected, on the one hand, to a brake pedal and, on the other hand, to a piston of a brake master cylinder, brake pressure being generated in at least one working space as a result of movement of the piston in the brake master cylinder, wherein the brake linkage comprises two rods adapted to be relatively movable in relation to one another within a predetermined range of movement, a switchable blocking device for selectively blocking the relative movement between the two rods being provided.

10. A brake linkage according to Claim 9, wherein the two rods extend coaxially relative to one another, one of the two rods being hollow-cylindrical at one end, and the other of the two rods being guided relatively movably with its end in the hollow-cylindrical end.

11. A brake linkage according to Claim 9, wherein the relative movement between the two rods takes place counter to the effect of a return spring.

12. A brake linkage according to any one of Claims 9 to 11, wherein the switchable blocking device is transferred into a switching position blocking the relative movement of the two rods in relation to one another, when the stroke of the piston in the brake master cylinder exceeds a switching threshold value.

13. A brake linkage according to Claim 12, wherein the switching operation of the blocking device takes place by means of a positive control.

14. A brake linkage according to Claim 13, wherein the positive control is formed by arranging on a housing at least partially surrounding the two rods a switching boss which, when the stroke of the piston exceeds the switching threshold value, transfers a blocking pawl, held in its open position by a holding spring, into its closed position, a mechanical connection between the two rods which is capable of being at least pressure-stressed being made when the blocking pawl is in the closed position.

15. A brake linkage according to Claim 14, wherein the blocking pawl and holding spring are fastened either to the first rod or to the second rod, and a detent nose is formed on the second rod or on the first rod respectively, a nonpositive connection between the detent nose and the blocking pawl being made in the closing position.

16. A brake linkage according to any one of Claims 9 to 12, wherein a control device determines, from signals supplied to it, that a blocking of the relative movement between the two rods is necessary, and corresponding control signals from the switching device trigger a necessary switching operation of the blocking device.

17. A brake linkage according to any one of Claims 9 to 16, wherein the blocking effect of the switchable blocking device acts only in the direction in which a relative movement between the rods which shortens the total length formed by the two rods is blocked, whereas an opposite relative movement within the scope of the predetermined range of movement can be executed without being blocked.

18. A brake linkage according to any one of Claims 1 to 17, wherein a fluidic connection between the at least one working chamber of the brake master cylinder and wheel-brake cylinders is such as to be capable of being interrupted in each case by means of a shut-off valve, a blocking of the relative movement between the two rods taking place when a closing of the shut-off valve is discontinued in the event of an actuation of the brake.

19. A brake master cylinder with a plunger piston loaded by a brake linkage, substantially as described herein with reference to, and as illustrated in, the accompanying drawings.

20. A brake linkage adapted to be connected, on the one hand, to a brake pedal and, on the other band, to a piston of a brake master cylinder, substantially as described herein with reference to, and as illustrated in, the accompanying drawings.