GB2148431A

GB2148431A - Quick-fill master cylinder

Info

Publication number: GB2148431A
Application number: GB08426437A
Authority: GB
Inventors: Hans-Dieter Reinartz; Helmut Steffes
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co oHG
Priority date: 1983-10-21
Filing date: 1984-10-19
Publication date: 1985-05-30
Also published as: GB8426437D0; GB2148431B; DE3338247C2; FR2553725A1; FR2553725B1; DE3338247A1

Abstract

A tandem master cylinder (1) which operates with a filling stage consists of two pressure chambers (10) and (11) separated by a piston (9) in a pressure-sealed manner. A stepped input piston (6) has a portion of larger diameter (25), a forwardly extending sleeve (46) and a portion of smaller diameter (24). A pressure relief valve (48) and a second valve (49) are guided within the stepped piston (6). The two valves (48, 49) together form a sealing seat (59). On displacement of the input piston (6), initially the valve piston (65) seats on the ball (56) closing valve 49 and the piston head (25) acts as a quick fill piston, fluid being drawn into the chamber (31) through an annular recuperation valve (43). At the pressure set by the spring (62), the relief valve (48) opens reducing the effective area of the piston head (25) by allowing fluid to pass from the chamber (10) to the chamber (31) through bore (33), the piston now pressurising the chamber (10) and by the piston (9) the second pressure chamber (11). <IMAGE>

Description

SPECIFICATION Master cylinder This invention relates to a master cylinder with a variable transmission ratio, in particular for brake systems of automotive vehicles, which master cylinder has a bore which is provided in a housing for the reception of a stepped piston, which piston sealingly slides with a larger diameter portion in the bore, which larger diameter portion is adjacent an end of the stepped piston, which end and the bore confines in part a first pressure chamber connectible in use with wheel brakes, and which stepped piston with a smaller diameter portion forms another chamber which, laterally, is confined by an annular wall of the stepped piston and by an end wall of the housing and which, radially, is confined by the bore, in the release position of the stepped piston said other chamber communicating with a supply reservoir via a first channel, closable by a first valve upon the operation of the stepped piston, and with the first pressure chamber via a second channel, closable by a second valve, and said other chamber being pressurisable by the pressure of the first pressure chamber via a pressure retaining valve provided in the stepped piston and being opened by the pressure of the first pressure chamber against the force of a valve spring, the pressure retaining valve consisting of a closure member movable within the stepped piston and of an end part with a sealing seat.

In a known master cylinder of this type (DE OS = German Printed Patent Application No.

22 50 392), in the release position of the master cylinder, a metering valve (the pressure retaining valve) separates the first pressure chamber from the other chamber. However, so as to have both the first pressure chamber and the other chamber always communicating with the supply reservoir in this position, in the housing, on the one hand, a tilting valve is provided for establishing the flow medium communication from the supply reservoir to the other chamber. On the other hand, at the other end of the piston, a sealing arrangement is provided for the flow medium communication from the other chamber to the first pressure chamber. Because the valves are spaced apart from each other in the master cylinder, there results an expensive and complicated construction. The numerous sealing points also increase the risk of leakage.

It is thus an object of this invention to provide a master cylinder of the type referred to above which features a simple structure and requires a small constructional expense, while needing as few sealing points as possible. Further, the construction of the master cylinder should be extremely short, with the weight being as low as possible.

According to this invention, this object is solved in that the sealing seat is formed by a valve piston movable in respect of the closure member and of the stepped piston, which valve piston, by means of a catch device operable by the stepped piston, keeps the pressure retaining valve open in the release position and closes it in the operating position. By means of the integration of the second valve into the pressure retaining valve, the master cylinder will not only do with but a few components, but, according to this invention, only one sealing seat will be required which, on the one side, forms the seal of the second valve and which, on the other side, forms the seal of the pressure retaining valve.

Thanks to this design, the piston seal may be a simple gasket of the type common in master cylinders and used there in the majority of cases. In combination with the valve member, the valve piston substitutes for the expansion bore arrangement known in master cylinders.

The expansion bore arrangement namely is disadvantageous in that in case of a rapid succession of operations a particularly strong wear will make itself felt at the piston seal which may result in premature failures of the master cylinders.

In order to ensure that the second valve guarantees an especially reliable operation, with its manufacture being simple, according to this invention a stop fastened at the stepped piston is provided between the closure member and the valve piston and the closure member and the valve piston are resiliently biassed in the closing direction of the pressure retaining valve, the sealing seat of the pressure retaining valve being provided respectively on the valve piston side of the stop or beyond thereof and axial play being provided between the stop and the two closure elements. Thanks to this simple arrangement, in the release position of the master cylinder, the closure member will be pressed by the force of the spring against the stop while the catch device lifts the valve piston off from the sealing seat against the force of the second compression spring.In the operating position of the master cylinder, the catch device releases the movement of the valve piston towards the sealing seat so as to enable the valve piston to abut on the sealing seat. Upon a further operation, a pressure may build up in the first pressure chamber, which pressure will displace the valve piston until it will come to rest at the stop. In doing so, the closure member is moved away from the stop.

A particularly simple guidance as well as an axial limitation of the movement of the closure member and of the valve piston will be achieved if the closure member and the valve piston lie on the centre line of the bore and in that the closure member is guided within a pocket bore provided within the stepped pis ton while the valve piston is held in a respective bore adjacent the open end of the pocket bore and formed by the inside surface of a sleeve connected with the first pressure-chamber-side end of the stepped piston. This arrangement, likewise, renders the fitting of the pressure retaining valve at the stepped piston easy.

So as to adjust a small axial misalignment between the pocket bore and the bore in the sleeve which possibly might result from fitting the sleeve on the stepped piston or from manufacturing tolerances at the closure member the sealing seat is formed by a ball, and at the valve piston the sealing seat is formed by an annular surface.

A simple channel guidance is formed at the pressure retaining valve if the valve piston rests at the wall of the respective bore in a pressure-sealed manner and has a central passage and, radially outside the sealing surface, passages are provided at the closure member.

A particularly simple catch device is realised at the second valve in an advantageous manner in that at the end confronting the sealing seat a compression spring is biassed between the sleeve and the valve piston and in that the central passage is penetrated by a tappet which in the release position of the master cylinder, via a stop stationary with respect to the housing in the release position at least, lifts the valve piston off from the closure member against the force of the compression spring. Thanks to this arrangement, a particularly simple valve operation will be established which, at all times, will work reliably and which in the range of the released and closed positions of the second valve is controlled in a travelsensitive manner by the stepped piston.

According to a further development, the valve member is sealed in respect of the wall of the respective bore and the sleeve is connected with the stepped piston in a pressuresealed manner. By means of this measure the flow medium may respectively flow along, or be separated at, the valve seat only, at the same time the valve piston being reliably pressed against the stop due to the pressure prevailing in the first pressure chamber.

An especially simple catch device in a master cylinder with a single pressure chamber will be achieved in that at the end of the first pressure chamber, at the bottom, a pocket bore is provided, which is open towards the first pressure chamber and in which one end of the tappet is telescopically slidable, and in that the movement of the tappet is limited within the pocket bore by the stop stationary with the housing. In this arrangement, the tappet will permit but a certain maximal distance of the valve piston in respect of the stop stationary with the housing. Thus, via the catch device, the valve piston is bound to the housing. approach of the valve piston up to the stop stationary with the housing is possible due to the telescopic sliding of the tappet into the pocket bore of the master cylinder.

The inventive master cylinder, however, also may be designed as a tandem master cylinder (with two pressure chambers). In this arrangement, according to this invention, the portion of the bore receiving the stepped piston is followed by a second bore portion of smaller diameter in which a second piston is disposed and separates the first pressure chamber from a second pressure chamber.

Further, at the second piston a pocket bore is provided which is open towards the first pressure chamber and in which one end of the tappet is telescopically slidable. The tappet is held within the pocket bore by the stop which is fast with the second piston, and in the initial position of the master cylinder the second piston supports itself at the housing. By this arrangement, in a tandem master cylinder, the tappet is guided in a simple manner within the second piston. And it is both ensured that, on the one hand, the valve piston will open the pressure retaining valve in the initial position of the master cylinder and that, on the other hand, the second piston without a retracting spring will abut on the stop formed fast with the housing, i.e. that it will always adopt a defined initial position.

So as to achieve an especially short construction of the tandem master cylinder and a weight saving resulting therefrom, it is advantageous that between the first and second pressure chambers at the housing two gaskets are provided which slidingly rest at the second piston and which operate as non-return valves, opening in the direction of the associated pressure chamber, and that between the two gaskets a channel ends in the second bore portion, which channel communicates with the supply reservoir in use, with the diameter of the second bore portion being slightly larger than the diameter of the second piston.In this arrangement, in which the second piston must be designed as a plunger piston, a pressure drop in a pressure chamber will be indicated by the pedals being pushed down until abutment of the 'defective piston' on the 'intact piston', thus driver will immediately know that his brake system is out of order. However, the driver will be able to brake the vehicle by means of the intact brake circuit. The two gaskets designed as non return valves will enable a further supply of brake fluid to one or both pressure chambers if required there because of lining wear.

An embodiment of the invention is repre sented in longitudinal section in the accom panying drawing and will be described in more detail in the following:A tandem master cylinder 1 comprises a housing 2 and a stepped bore 3 extending in the longitudinal direction of the housing 2. At one end, the right hand end 4 as viewed in the drawing, said bore 3 is pressure-sealed by a sleeve 5 screwed into the bore 3 and by a stepped piston 6 penetrating the sleeve 5. At its other (left hand) end 7, said bore 3 is sealed by a cover 8 caulked with the bore 3 in a pressuresealed manner. Between the stepped piston 6 and the rear end 7, a second piston 9 is provided in the bore 3, said second piston 9 dividing the bore 3 into a first pressure cham ber 10 and into a second pressure chamber 11.The two pressure chambers 10, 11 each are connected with a respective vehicular brake circuit (not shown in the drawing). The stepped piston 6 is guided in a bore section of larger diameter 1 2. The second piston 9 is guided in a bore section of smaller diameter 13.

With its right hand end formed by a cylindrical shaft 14 and representing the actual piston diameter of the stepped piston 6 in the pressure phase, the stepped piston 6 protrudes from the housing 2 of the tandem master cylinder 1. By means of a retaining ring 15, a sheet metal plate 1 6 is fastened at the right hand end of the cylindrical shaft 14.

A compression spring 1 7 is supported at the sheet metal plate 1 6. The other end of the compression spring 1 7 rests on the housing 2. The compression spring 1 7 serves to move the stepped piston 6 and the second piston 9 into their initial positions, which are those represented in the drawing, after the completion of a braking operation. The cylindrical shaft 1 4 has a recess 1 8 into which a brakepedal-operable actuating tappet (not illustrated in the drawing) projects, resting at the spherical end 1 9 thereof. The actuating tappet, e.g.

may be part of a working piston of a brake booster or part of a brake pedal. At the housing 2, a flange 20 is provided which serves for attachment to e.g. a brake booster housing or to a splash wall of an automotive vehicle.

A seal 21 seals the sleeve 5 in respect of the housing 2, a seal 22 and a following gasket 23 sealing the sleeve 5 in respect of the cylindrical shaft 14. The seal 22 substantially performs the function of fending-off dirt whereas the seal 23 serves to fend off flow (pressure) medium. The seals 21, 22, and 23 are stationarily fastened within the sleeve 5.

Thus, the stepped piston 6 performs the function of a plunger piston. The cylindrical shaft 14 forms a portion of smaller diameter 24, the pressure-chamber-side (left hand) end forming the portion of larger diameter 25, of the stepped piston 6. Fastened on the portion of larger diameter 25 is a non-return valve in the form of a gasket 26, said non-return valve opening in the direction of the first pressure chamber 10.

Referring to the drawing, the larger diameter 1 2 of the bore 3 is followed to the right by an annular step 27 which expands the bore and in which a first channel 28 is ending. Said first channel 28 extends into the chamber 29 which serves for the reception of a supply reservoir's outlet socket (not illustrated in the drawing). Towards the right hand end 4, the step 27 is followed by a bore section 30 forming a chamber 31, together with the piston portion of smaller diameter 24. On the one side, the chamber 31 is confined by the step 27 and by an annular wall 32 provided at the stepped piston 6 and, on the other side, it is confined by the left hand faces 33 of the sleeve 5 and by the gasket 23.

The portion of smaller diameter 24 of the stepped piston 6 is surrounded by an annular disk 34 whose cross-section essentially is Ushaped and the opening of which U-section of the annular disk 34 is directed towards the first pressure chamber 1 0. In the illustrated release position of the tandem master cylinder 1, the radially inside leg 35 of the annular disk 34 rests at the annular wall 32, thus keeping a seal 37 at a small distance relative to the step 27. Said seal 37 is fastened radially inside and outside at the outer leg 36.

Thus, in this position, the first channel 28 which ends in an axial groove 38 which is open towards the seal 37 communicates with the chamber 31, the outer leg 36 of disk 34 slightly projecting into the axial groove 38 so that the annular disk 34 will always be guided transversely in respect of the longitudinal axis 39 of the tandem master cylinder 1.

One left hand face 33 of the sleeve 5 is abutted by a disk 41 having axial openings 40. The axial openings 40 serve to bleed the chamber formed between the gasket 23 and the disk 41. Between the disk 41 and the annular disk 34 a first valve spring 42 is biassed which upon the operation of the stepped piston 6 will bring about a closure of a first valve 43 formed by the parts 34, 37, 38, and 42 already described.

From the chamber 31 to the right of the back wall 32, a channel 44 leads radially from the outside into the cylindrical shaft 14 of the stepped piston 6. Said channel 44 ends in a pocket bore 45 extending concentrically in respect of the longitudinal axis 39 of the stepped piston 6. The pocket bore 45, expanding by steps, extends from the spherical end 1 9 up to the pressure-chamber-side end of the stepped piston 6. Following the pressure-chamber-side end of the stepped piston 6 is a cup-shaped sleeve 46 projecting into the pressure chamber 10. By means of its rim 47, said sleeve 46 is caulked in a pressure-sealed manner at the pressure-chamberside end of the stepped piston 6. The chamber formed by the pocket bore 45 and the sleeve 46 serves to receive a pressure retaining valve 48 and a second valve 49.

The pressure retaining valve 48 is formed by a closure member 50 consisting of a piston 51 with a pin 52 which follows the piston 51 and extends towards the right hand end 4 and of a ball 56 fastened at the front end of the piston 51 in a recess 55. The pin 52 is sealed in respect of the pocket bore 45 by means of a seal 53; thus a vacuum chamber 54 communicating with the space of the recess 1 8 is separated in a pressure-sealed manner from a pressure retaining valve chamber 57 formed by the piston 51 and the pocket bore 45, which pressure retaining valve chamber 57 communicates with the chamber 31 via the channel 44.

Along the outside circumferential surface of the closure member 50 passages 58 are extending which are connecting a sealing seat 59 with the pressure retaining valve chamber 57. In the illustrated release position of the tandem master cylinder 1, due to a valve spring 62, the left hand end of the closure member 50 abuts on a stop 60 designed as disk. The rim 47 and the caulking 61 keep the stop 60 stationarily in this position at the stepped piston 6. The valve spring 62 is provided in the pressure retaining valve chamber 57 and supports itself at the closure member 50, on the one hand, and at the stepped piston 6 via a disk 63, on the other hand. The disk 63 chiefly serves for the axial limitation of the seal 53.

Just in front of the sealing seat 59 of the ball 56 there is provided an annular surface 64 of a valve piston 65. Said annular surface 64 cooperates with the sealing seat 59. Via its radially outer surface area, the valve piston 65 is guided within cylindrical wall 66 of the sleeve 46 and sealed by a gasket 67. The valve piston 65 is biassed to the right by means of a compression spring 68. The compression spring 68 is held between the end of the valve piston 65 and the left hand end 69 of the cup-shaped sleeve 46. The valve piston 65 and the end 69 are each penetrated by a central passage 70 and 71, respectively. The central passages 70 and 71 connect the pressure chamber 10 with the sealing seat 59 and form together with the passages 58 a second channel.

The closure member 50 and the valve piston 65 form the second valve 49. Further, belonging to the second valve 49 is the catch device 72 consisting of a tappet 73 which, on the one hand, penetrates the central passage 70 and abuts with its annular collar 74 on an annular face 75 of the valve piston 65. At the other end of the tappet 73 a disk 104 is fastened which is guided in a pocket bore 76 of the second piston 9, which pocket bore 76 is open towards the pressure chamber 10. In the release position of the master cylinder 1, the disk 104 abuts on a stop formed by a retaining ring 77 fastened in an annular groove of the pocket bore 76. In the pressure chamber 10, a compression spring 78 is extending from the stepped piston 6 to the second piston 9. In this position, the compression spring 78 urges the two pistons 6 and 9 apart.Their maximal distance is defined by the distance between the annular collar 74 and the disk 104. The tappet 73 couples the two pistons 6 and 9. At the circumference of the disk 104 an axially extending channel 79 is provided which connects the pressure chamber 10 with a chamber 80 formed in the pocket bore 76 between the disk 104 and the closed end of the second piston 9. Thereby it is possible to bleed the chamber 80. The pressure in said chamber 80 will always be the pressure of the first pressure chamber 10.

On the surface area of the second piston 9, two gaskets 81, 82 rest in a pressure-sealed manner, the gasket 81 confining the pressure chamber 10 and the gasket 82 confining the pressure chamber 11. Between the two gaskets 81 and 82, radially from the outside, a channel 83 is extending via an annular groove 85 into the smaller diameter portion 1 3 of the bore 3. Said channel 83 communicates with a chamber 84. Said chamber 84 serves to receive a connecting socket of a supply reservoir, said socket not being illustrated in the drawing. As the smaller diameter portion 1 3 is slightly larger than the diameter of the second piston 9 a flow medium communication exists from the supply reservoir towards the two gaskets 81. 82. The two gaskets 81 and 82 being designed as non-return valves.

if required, flow medium may flow from the supply reservoir into the pressure chambers 10, 11.

The end of the second piston 9 facing the pressure chamber 11 ends with an annular collar 86 which reaches behind a radially inward directed leg of a sleeve 87 having an essentially Z-shaped cross-section. The cylindrical portion of the sleeve 87 has a diameter slightly larger than the largest diameter of the second piston 9. The cylindrical portion extends into the pressure chamber 11. At its end, a radially outwardly expanding annular collar 88 is provided which reaches behind a radially inwardly directed portion 89 of a valve disk 90 having a T-shaped cross-section with the leg of the T directed parallel to axis 39. The foot of the leg of the T-shaped valve disk 90 projects into an axial annular groove 91 starting at a step 92 of the cover 8. This is to ensure the guidance of the valve disk 90.

An annular portion 93 of the leg of the Tshaped valve disk 90 is provided with seals 94 and 95, respectively. The seals 94, 95 support themselves at the cross beam 96 of the valve disk 90 and are glued or vulcanised, respectively, to the wall of the annular portion 93. A compression spring 97 resting against the housing 2 and at the valve disk 90 ensures that, upon the operation of the second piston 9, the operation of the valve disk 90, and hence of valve arrangement 98, will be exact and free from play. The step 92, together with the left hand ends of the seals 94 and 95, forms the sealing seat of the valve arrangement 98.

A channel 99 communicating with the chamber 84 ends radially from the outside in the bottom of the axial annular groove 91. At the transition from the cover 8 towards the housing 2, seals 100, 101 seal the channel 99 in respect of the atmosphere and in respect of the second pressure chamber 11. The ends of the channels 99 horizontally and vertically extending out of the housing 2 are pressure-sealed by means of balls 102, 103.

The mode of operation of the inventive tandem master cylinder 1 will be described in more detail in the following:ln the represented release position of the tandem master cylinder 1, the first and the second valves 43 and 49 as well as the valve arrangement 98 are open, a fluid communication existing from the chamber 29 to the first pressure chamber 10 and from the chamber 84 to the second pressure chamber 11. Upon the initiation of a braking operation, by the actuation of the stepped piston 6 to the left in the drawing, the second piston 9 will be advanced by the compression spring 78.In doing so, at first the first valve 43 will be caused by the stepped piston 6 to adopt its closing position and the second piston 9 will cause the valve arrangement 98 to adopt its closing position, thus no longer any flow medium communication existing from the chamber 29 and from the chamber 84 to the first and second pressure chambers 10, 11. As in this phase of operation the stepped piston 6 and the second piston 9 are moved as a unit, the second valve 49 as well as the catch device 72 will remain in their initial or rather open positions.

Upon further operation, at first a small pressure will build up in the second pressure chamber 11. As a result thereof, the second piston 9 now will be displaced more slowly than the stepped piston 6. Thereby it will be achieved that the catch device 72 as well as the valve piston 65 will move more slowly relative to the movement of the two pistons 6 and 9. Thus the stepped piston 6 and the pressure retaining valve 48 will approach the valve piston 65 until the ball 56 will rest at the sealing seat 59 of the valve piston 65.

The first pressure chamber 10 will be closed towards the chamber 31, the filling operation starting.

The surface effective for the filling operation is comprised by the annular surface resulting from the circular surface of the portion of larger diameter 25 less the circular surface of the portion of smaller diameter 24. Upon a further operation of the stepped piston 6, due to the filling stage, a relatively large amount of pressure medium will be supplied to the wheel brakes connected with the first pressure chamber 10, thus a fast application of these brakes resulting and a pressure building up in the first pressure chamber 10 in a very quick time. The demand for volume resulting in the chamber 31, upon the advancing movement of the stepped piston 6, will be compensated by the automatic opening of the first valve 43 against the force of the first valve spring 42.

The first valve 43 will open due to the pressure difference acting on the first valve 43.

The pressure difference will be formed on the one side by the vacuum forming in the chamber 31 and, on the other side, by the atmospheric pressure.

Upon a further pressure build-up, after the valve piston 65 has come to rest in a pressure-sealed manner at the closure member 50, the pressure prevailing in the first pressure chamber 10 will act on the valve piston 65 which is displaceable within the sleeve 46.

Thus the force resulting therefrom will move the valve piston 65 and the closure member 50 to the right in the drawing until the valve piston 65 will have reached the stop 60.

Upon a further pressure increase, now, there will be an increase in the closure member's 50 opening force directed to the right in the drawing. The opening force exceeding the force of the valve spring 62, pressure medium will flow from the first pressure chamber 10 via the central passage 70, the sealing seat 59, and the passages 58 to the chamber 31.

As the surface of the pin 52 is smaller than the effective surface of the ball 56 radially lying within the area of the sealing seat 59, the force of the valve spring 62 will cause the closure member 50 to move again to the left and to close the pressure retaining valve 48.

The filling operation will start with the first opening of the pressure retaining valve 48 and will end in that position in which the pressure retaining valve 48 will be permanently open, the pressure of the chamber 31 then corresponding to the pressure of the first pressure chamber 10. The pressure retaining valve 48 thus having reached its change-over point, by means of the pressure retaining valve 48 there will be a steady adaptation of the pressure of the chamber 31 to the pressure of the first pressure chamber 1 0. This slowly increasing pressure build-up in the chamber 31 will go on until the force resulting at the pin 52 will equal the force of the valve spring 62.This upper changeover point having been reached, the pressure retaining valve 48 will remain in its open position and the filling operation will be completed as now at the wall 32 there will be the same pressure as at the corresponding pressure-chamber-side annular surface of the stepped piston 6. The gasket 26 will become ineffective, the gasket 23 taking its place and, together with the portion of smaller diameter 24 of the stepped piston 6, now forming the working pressure in the pressure chamber 10 during the braking operation.

Upon release of the brake application, the stepped piston 6 and the second piston 9 will return into their initial positions. As soon as the pressure in the first pressure chamber 10 will have reached a value smaller than the pressure needed for the open position of the closure member 50, the closure member 50 will move to the left and will close the pressure retaining valve 48. At this moment, the filling stage again will start and act in the reverse direction by now supplying the pressure medium at an increased rate from the first brake circuit into the first pressure chamber 10, the movement of the stepped piston 6 being the same.

Upon a further pressure reduction, due to the higher pressure in the chamber 31, the valve piston 65 will be moved to the left and the pressure retaining valve 48 will be opened for the purpose of a pressure compensation.

This operation will be repeated until the catch device 72 will no longer allow any further displacement of the valve piston 65 to the right, the annular collar 86 resting at the housing 2 and the sleeve 104 simultaneously resting at the stop 77 and thereby the pressure retaining valve 48 being opened entirely.

In the release position of the master cylinder 1, due to the force of the compression spring 68, the second piston 9 will thus support itself via its annular collar 86 at the parts 87 and 105 supporting themselves at the housing 2. Thus the catch device 72, likewise against the force of the compression spring 68, will keep the valve piston 65 rigid in respect of the housing. This resilient prestress of the parts 9, 72, and 65 will be maintained by the stepped piston 6 biassed to the right by the compression spring 1 7. In this operating position at the same time the first valve 43 and the valve arrangement 98 will be opened by the respective pistons 6 and 9, thus the pressure chambers 10 and 11 communicate again with the supply reservoir.The compression spring 1 7 urges the stepped piston 6 into its initial position, thus likewise the second piston 9 reaches its initial position by means of the catch device 72.

Upon failure of one brake circuit, e.g. of the brake circuit connected with the first pressure chamber 10, the stepped piston 6 will approach the second piston 9 until it will abut with its front face 106 on the first pressurechamber-side end of the second piston 9. In doing so, the front portion of the pressure retaining valve 48 will move into the pocket bore 76. Upon a further operation, now, a pressure may build up in the second pressure chamber 11 which at least will supply pressure medium to the brake circuit connected with the second pressure chamber 11.

Upon failure of the brake circuit connected with the second pressure chamber 11, the two pistons 6 and 9 will move evenly to the left. The pressure retaining valve 48 will not close until the second piston 9 will have been caused by the force of the compression spring 78 to rest at the bottom of the cover 8. At first, pressure medium will be supplied from the first pressure chamber 10 into the first brake circuit. This operation will be effected together with the filling stage. Upon reaching the filling stage pressure, the pressure retaining valve 48 will slowly open and reduce the filling stage steadily until only the pressure stage will be in operation. This operation will thus correspond to the mode of operation of the first pressure chamber 10, with the brake system being intact, while the defect second pressure chamber 11 will remain unpressurised.

Claims

1. A master cylinder with a variable transmission ratio, which master cylinder has a bore which is provided in a housing for the reception of a stepped piston. which piston sealingly slides with a larger diameter portion in the bore, which larger diameter portion is adjacent an end of the stepped piston, which end and the bore in part a first pressure confine chamber connectible in use with wheel brakes, and which stepped piston with a smaller diameter portion in part forms another chamber which, laterally, is confined by an annular wall of the stepped piston and by an end wall of the housing and which, radially, is confined by the bore, in the release position of the stepped piston said other chamber communicating with a supply reservoir via a first channel, closable by a first valve upon the operation of the stepped piston, and with the first pressure chamber via a second channel, closable by a second valve, and said other chamber being pressurisable by the pressure of the pressure chamber via a pressure retaining valve provided in the stepped piston and being opened by the pressure of the first pressure chamber against the force of a valve spring, the pressure retaining valve consisting of a closure member movable within the stepped piston and of an end part with a sealing seat, characterised in that the sealing seat (59) is formed by a valve piston (65) movable in respect of the closure member (50) and of the stepped piston (6), which valve piston (65), by means of a catch device (72) operable by the stepped piston (6), keeps the pressure retaining valve (48) open in the release position and closes it in the operating position.

2. A master cylinder as claimed in claim 1, characterised in that a stop (60) fastened at the stepped piston (6) is provided between the closure member (50) and the valve piston (65); in that the closure member (50) and the valve piston (65) are resiliently biassed in the closing direction of the pressure retaining valve (48); in that the sealing seat (59) of the pressure retaining valve (48) is provided respectively on the valve piston side of the stop (60); and in that axial play is provided be tween the stop (60) and the closure member (50) and the valve piston (65).

3. A master cylinder as claimed in claims 1 and 2, characterised in that the closure member (50) and the valve piston (65) lie on the centre line (39) of the bore (3) and in that the closure member (50) is guided within a pocket bore (45) provided within the stepped piston (6) while the valve piston (65) is held in a respective bore (66) adjacent the open end of the pocket bore (45) and formed by the inside surface of a sleeve (46) connected with the first pressure-chamber-side end of the stepped piston (6).

4. A master cylinder as claimed in claim 3, characterised in that, at the closure member (50) the sealing seat (59) is formed by a ball (56), and at the valve piston (65) the sealing seat (59) is formed by an annular surface.

5. A master cylinder as claimed in claims 3 and 4, characterised in that the valve piston (65) rests at the wall of the respective bore (66) in a pressuresealed manner and has a central passage (70) and in that, radially outside the sealing seat (59), passages (58) are provided at the closure member (50).

6. A master cylinder as claimed in any one of the preceding claims, characterised in that at the end confronting the sealing seat (59) a compression spring (68) is biassed between the sleeve (46) and the valve piston (65) and in that the central passage (70) is penetrated by a tappet (73) which in the release position of the master cylinder (1), via a stop (77) stationary with respect to the housing in the release position at least, lifts the valve piston (65) off from the closure member (50) against the force of the compression spring (68).

7. A master cylinder as claimed in any one of the preceding claims. characterised in that the valve piston (65) is sealed in respect of the wall (66) of the respective bore and in that the sleeve (46) is connected with the stepped piston (6) in a pressuresealed manner.

8. A master cylinder as claimed in claim 6, characterised in that, at the end of the first pressure chamber (10) opposite said stepped piston (6), a pocket bore is provided which is open towards the first pressure chamber (10) and in which one end of the tappet is telescopically slidable and in that the movement of the tappet is limited within the pocket bore by the stop (77).

9. A master cylinder as claimed in claim 6, characterised in that the portion of the bore (3) receiving the stepped piston is followed by a second bore portion (13) of smaller diameter in which a second piston (9) is disposed and separates the first pressure chamber (10) from a second pressure chamber (11),; in that at the second piston (9) a pocket bore (76) is provided which is open towards the first pressure chamber (10) and in which one end (104) of the tappet (73) is telescopically slidable; in that within the pocket bore (76) the tappet (73) is held by the stop (77) which is fast with the second piston; and in that, in the initial position of the master cylinder (1), the second piston (9) supports itself at the housing (2).

10. A master cylinder as claimed in claim 9, characterised in that, between the first and second pressure chambers (10, 11), at the housing (2) two gaskets (81, 82) are provided which slidingly rest at the second piston (9) and which each operate as non-return valves opening in the direction of the associated pressure chamber, and in that between the two gaskets (81, 82) a channel (83) ends in the second bore portion (13), which channel (83) communicates with the supply reservoir in use, with the diameter of the second bore portion (13) being slightly larger than the diameter of the second piston (9).