FR2602835A1 - BRAKE EFFORT HYDRAULIC AMPLIFIER - Google Patents

Abstract

IN A HYDRAULIC EFFORT AMPLIFIER 1 IN PARTICULAR INTENDED FOR AN AUTOMOTIVE VEHICLE BRAKING SYSTEM WITH ANTI-LOCK REGULATION, A CRANKCASE 4 HOUSES A PRESSURE CHAMBER 48 WHICH IS ARRANGED BETWEEN AN AMPLIFIER PISTON 5 AND A COAXIAL PISTON 20 AND WHICH CAN BE CONNECTED TO A SOURCE OF PRESSURE LIQUID 11 THROUGH AN INPUT CHAMBER 3 DELIMITED BY A CIRCUMFERENTIAL PART OF THE AMPLIFIER PISTON 5, THE CRANKCASE 4 AND TWO ANNULAR SEALS 9, 10, PER L 'INTERMEDIATE OF AN AXIAL DRILLING 19 COMMUNICATING WITH THE INLET CHAMBER 8 AND THROUGH A SUPPLY VALVE 22, AND WHICH CAN BE CONNECTED TO THE SUPPLY TANK 17 THROUGH THE DRAIN VALVE 24. THE TWO VALVES 22 , 24 ARE DESIGNED IN THE FORM OF SEAT VALVES ARRANGED IN THE AMPLIFIER PISTON AND WHICH CAN BE ACTUATED DEPENDING ON THE POSITION OF THE DRIVE PISTON 20. TO REDUCE ITS REACTION AND DRIVING FORCE, THE AMPLIFIER PISTON 3 IS DESIGNED IN THE FORM OF A TWO-STAGE PISTON OF WHICH THE SMALL STAGE 7 DELIMITS THE INLET CHAMBER 8, THE SUPPLY VALVE AND THE DISCHARGE VALVE 22, 24 BEING LOCATED ONE BEHIND THE OTHER AND COAXIALLY WITH THE 'PISTON AXIS, AND THE PRESSURE CHAMBER 48 BEING INTERCALED BETWEEN THE LARGE PISTON STAGE 6 AND THE INLET CHAMBER 8.

Description

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  The present invention relates to a b iraliune effrt aplificater, intended in particular for a motor vehicle braking system, in particular with anti-lock regulation, in which a casing houses a pressure chamber which is disposed between an amplifier piston and a coaxial actuating piston and which can be connected to a source of pressure liquid via an inlet chamber delimited by a circumferential part of the amplifier piston, the casing and two annular seals, via an axial bore communicating with the inlet chamber and via a supply valve, and which can be connected to a supply tank through a discharge valve, these two valves being designed in the form of seat valves arranged in the amplifier piston and operable according to

  the position of the actuating piston.

  In a known amplifier of this type (patent application

  German 29 09 685), the inlet chamber for the pressure liquid is arranged on the circumference of the amplifier piston which has the largest diameter. In this case, the two joints of the inlet chamber are constantly subjected to the high inlet pressure and therefore produce a friction force of corresponding level which requires an even higher actuating force.

  To reduce this actuating force, it is known to install a pressure relief valve between the source of pressure liquid and the amplifier (German patent application 32 18 194) in order to reduce the inlet pressure to the minimum value. required. However, this pressure relief valve is an accessory member which is not absolutely essential for the operation of the amplifier and which

  increases cost and sensitivity to failures.

  The object of the present invention is to provide an amplifier 30 of the type indicated above, in which the friction of the joints subjected to the input pressure is reduced without causing

additional charges.

  In accordance with the present invention, this object is achieved in that the amplifier piston is designed in the form of a two-stage piston 35, the small stage of which has a smaller outside diameter.

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  delimits the inlet chamber, in that the supply valve and the outlet valve are located one behind the other and coaxial with the axis of the pistons, and in that the pressure chamber is interposed between the large piston stage of larger diameter and the inlet chamber. This arrangement eliminates the overpressure valve on the inlet side. However, the force of the pressure which is exerted on the seals sealing the inlet chamber is lower due to the smaller diameter of the small stage of the piston, so that the friction of the seals is reduced thereby. as much when the amplifier piston is moved. The series connection of the supply valve and the discharge valve not only facilitates the production of a piston stage of smaller external diameter, but also guarantees that the amplifier piston is not actuated by eccentrically by the actuating piston through these valves, as is the case in the amplifier indicated above, so that the amplifier does not exhibit a tendency to

  jamming likely to increase the actuating force.

  Preferably, provision is made for an outlet chamber communicating with the supply reservoir over the entire stroke of the piston to be applied against the circumference of the small piston stage, at the end of the remote inlet chamber. of the large piston stage, the sealing of this outlet chamber being ensured by one of two annular seals and by a third annular seal. The leakage from the seal of the inlet chamber opposite the largest piston stage is thus evacuated in the direction of the supply tank, which simultaneously makes it possible to evacuate the pressure-regulated pressure liquid over the entire stroke of the work until closing

of the drain valve.

  In an amplifier for an anti-lock regulation motor vehicle braking system (also called ABS - anti-blocking or slip regulation system), in which the face of the amplifier piston opposite the pressure chamber rests against a coaxial working piston a master cylinder of the braking system, this working piston acting on a working chamber, being

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  mounted leaktight relative to the casing and delimiting, with the casing and this face of the amplifier piston, a second outlet chamber which communicates with the supply tank, a blocking valve actuated according to the pressure can be installed on the con5 connecting line leading from the second outlet chamber to the supply tank, and the pressure chamber of the amplifier can be connected to the control input of the blocking valve through a main valve actuated according to the slip and to the second outlet chamber when part of the stroke of the working piston has been traveled. In this arrangement, as long as no slippage occurs, the pressure liquid is evacuated from the second outlet chamber towards the supply tank through the blocking valve opened when the amplifier piston is actuated, and the working piston is driven by the supply of the amplifier piston. Conversely, when the amplifier piston is released, the pressure liquid can also escape freely from the second outlet chamber to reach the supply tank through the open blocking valve. However, as soon as a slip occurs and the amplifier piston is actuated, the main valve, which has been previously closed, is opened and the blocking valve is controlled by the outlet pressure of the main valve to adopt its closed position. Since the blocking valve is closed, the pressure liquid can no longer be discharged from the second outlet chamber, so that the working piston can no longer be moved by the amplifier piston and the pressure braking cannot increase. However, if the working piston has already run a good partial stroke before slippage occurs and the main valve opens, the pressure liquid can flow directly from the working chamber of the amplifier into the second bedroom

  output and can recall the amplifier piston, the second output chamber simultaneously acting as a return chamber and a new normal actuation of the brakes being possible immediately after the disappearance of the slip. Despite the secondary stroke 35 without pressure which is necessary for the amplifier piston to

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  put a pressureless evacuation of the leaking liquid which enters the first outlet chamber from the inlet chamber, thus obtaining a short overall length of the amplifier.

  In this regard, a liquid inlet & throttling bore 5 which is made in the casing and which communicates, inlet side, with the pressure outlet of the main valve, can, in particular, be connected, outlet side, to the second outlet chamber after having been covered by the working piston when the partial stroke has been covered. A connection between the outlet side of the main valve and the second outlet chamber is thus already established when the partial stroke of the working piston is completed and before slippage occurs. When a slip occurs, the return of the amplifier piston, after the opening of the main valve, can therefore be carried out without a pressure being suddenly established in the second outlet chamber as long as the blocking valve East

  closed by means of the main valve outlet pressure.

  To close the working chamber of the working piston by

  relative to the supply tank which communicates with the working chamber through the main valve in the event of actuation of the amplifier piston 20 as long as the liquid inlet hole is not covered by the working piston and / or its seal, a non-return valve is installed between the outlet of the main valve and the liquid inlet hole, this non-return valve allowing the pressure liquid to flow in the opposite direction towards the second chamber when the anti-lock regulation is triggered.

  Instead of providing a non-return valve, the liquid inlet hole can also enter an annular chamber when the working piston is in the rest position, this annular chamber being delimited by the working piston, by the casing and 30 by two annular seals arranged between the working piston and the casing. If the amplifier piston rests against the working piston via a coaxial pin, this pin must

  have a mushroom-shaped head. This mushroom-shaped head guarantees a coaxial power transmission from the piston to

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  plifier on the working piston, so that the working piston is not subjected to an eccentric load and is not likely to get caught. This will help reduce the actuation force.

  The various objects and characteristics of the invention will now be detailed in the description which follows, given by way of nonlimiting example, with reference to the attached figures.

  which represent: FIG. 1, a motor vehicle braking system with anti-lock regulation equipped with a hydraulic amplifier in accordance with the invention,

  - Figure 2, a detail of a variant of a part of the braking system according to Figure 1.

  In accordance with FIG. 1, a hydraulic amplifier 1 acts on a master cylinder 2 of a motor vehicle braking system 15 with anti-lock regulation. The hydraulic amplifier 1 comprises, in a stepped bore 3 of a casing in two axially divided parts 4, a stepped amplifier piston 5 comprising a large piston stage 6 and a small piston stage 7, the diameter of the piston stage 6 being higher than that of piston stage 7. 20 The small piston stage 7 defines, with the casing 4, an inlet chamber 8 which surrounds the small piston stage 7 and of which, moreover, axial sealing is ensured by two annular seals 9 and 10 disposed between the casing 4 and the piston stage 7. Pressure liquid is sent from a source of pressure liquid 25 11, which comprises a pump 13 downstream which is implanted

  non-return valve-14 and, in parallel, a pressure accumulator 15, the pump 13 being driven by a motor 12 and the suction side of the pump 13 being connected to the supply tank 17 via a filter 16, in the annular chamber 8 via a supply pipe 41 and an inlet opening 18 made in the casing 4. The small piston stage 7 comprises a coaxially stepped bore 19, in which an actuating piston 20 can slide axially and which communicates with the inlet chamber 8 by means of a radial bore 21. In the bore 19 is provided a supply valve 22 and in a bore

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  coaxial ge 23 of the actuating piston 20 is arranged a valve

  discharge 24. The two valves 22 and 24 are seat valves.

  The supply valve 22 is provided with a resiliently stressed ball-shaped closure member 25 which is pushed against an annular valve seat member 26 by elastic force and by the pressure liquid supplied through radial drilling. The discharge valve 24 also includes an elastic closing valve closing element 27 associated with a ball. The valve closing element 27 is pushed against an annular valve seat element 28 by the elastic force and by the pressure liquid sent into the bore 23 through the supply valve 22. It is secured & coaxially to a pusher 29 which opens the supply valve 22 as a function of the actuation of the actuation piston 26.

  The actuating piston 20 is mounted leaktight relative to the bore 19 of the small piston stage 7 by means of two annular seals 30 and 31. The annular seals 30, 31 also guarantee the axial sealing of an annular chamber 33 disposed between the circumference of the actuating piston 20 and the bore 19, this annular chamber 20 communicating with the bore 23 by means of a radial bore 32. The annular chamber 33 is connected to an outlet chamber 35 by through a radial bore 34 made in the casing 4, the outlet chamber 35 surrounding the small piston stage 7 and being delimited by the latter and by the casing 4. The seal 25 of the outlet chamber 35 in the axial direction is provided by

  the annular seal 10 and by another annular seal 36 which is only subjected to the low outlet pressure since the outlet chamber 35 communicates with the pressurized supply tank 17 via a bore 37, a sound opening 38 and a return line 39. In addition, the return line 39 communicates, through a pressure relief valve 40, with the supply line 41 which leads from the outlet of the source of pressure liquid 11 at the inlet opening 18.

  The actuating piston 20 is immobilized axially in the bore 19 by means of an elastic ring 42 and it is actuated at

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  using a brake pedal 43 via an actuating rod 44.

  The annular seal 36 is immobilized axially by means of an elastic ring 45 which, for its part, is immobilized axially by means of the radial flange of a sleeve 46 which pad periiaore the body of the motor vehicle by means of 'a spring of

pressure 47.

  The large piston stage 6 delimits a pressure chamber 48 on the sides of the small piston stage 7, this pressure chamber being connected to the chamber 51, disposed between the two valves 22 and 24 in the bore 19, through a bore 49 extending radially in the small piston stage 7 and through a bore 50 parallel to the axis. In addition, the pressure chamber 48 is connected to the inlet of a main electromagnetically controlled valve 54 via a radial bore 52 and a pipe 53. The valve

  main 54 is a three-way / two-position valve which, in the event of a slippage of a vehicle wheel, is actuated by means of a slippage monitoring circuit not shown, that is to say that it passes from the closed position shown to an open position 20 te.

  The face of the large piston stage 6 of the amplifier piston limits, on the one hand, a second outlet chamber 55 in the bore 3 and is applied, on the other hand, against a first working piston 56 of the master cylinder 2 by means of intermediate discs, the tightness of this master cylinder with respect to the bore 3 of the casing 4 being ensured by an annular seal 57. A guide pin 58 penetrates into coaxial blind holes made, on the one hand , in the face of the large piston stage 6 of the amplifier piston 5

  and, on the other hand, in the rear face of the working piston 56.

  The face of the working piston 56 delimits a working chamber 59 in the bore 3 of the casing 4 of one side, this working chamber being delimited on the other side by the rear face of a second "floating" working piston "60. A pressure spring 61 is disposed between the sides of the two working pistons 56 and 60, these c835 tees being turned towards one another.

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  The second working piston 60 is provided with a circumferential groove 62 which, with the casing 4, defines an annular chamber 63. The annular chamber 63 communicates with the working chamber 59 by means of a radial bore 64, an axial bore 65 5 and a central valve 66 disposed in the working piston 60.

  The closing element 67 of the central valve 66 is connected to the working piston 56 by means of a head bolt 68, so that it can move relative to the working piston, but it can be driven by the piston 56. The sealing of the working chamber 59 with respect to the annular chamber 63 is ensured by a cup

seal 69.

  The face of the second working piston 60 delimits a second working chamber 70 on one side, this working chamber being delimited, on the other side, by a seat element 71 of a central valve 72, the seal of the valve seat element 71 relative to the bore 3 being provided by a sealing cup 73.

  The seat element 74 of the central valve 72 can slide axially in the valve seat element 71 and is connected to the second working piston 60 by means of a head bolt 75 which can also slide in the piston of work, the closing element of

  valve 74 which can be driven by the working piston 60 by means of the head bolt 75.

  The sealing of the working chamber 70 relative to a third outlet chamber 76 is ensured by the sealing cup ity 73 and, relative to the annular chamber 63, by a sealing cup 77 which surrounds the piston of working 60. A pressure spring 78 disposed between the face of the working piston 60 and the valve seat element 71 moves the latter away from the working piston

one of the other.

  The outlet chamber 76 communicates with the supply tank 17 via a hole 79, an opening of

connection 80 and a pipe 81.

  The central valve 72 closes the working chamber 70 in the direction of the outlet chamber 76.

  In addition, the annular chamber 63 communicates with the reservoir.

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  see supply 17 via a radial housing bore 62, a line 83, the main valve 54, a line 84, a housing bore 85, a connection opening 86 and a

driving 87.

  The outlet chamber 55 communicates with the pipe 84 via a housing bore 88, a pipe 89, a blocking valve 90 actuated by pressure and designed in the form of a two-way valve channels / two positions, and of a pipe 91. In addition, the outlet side of the main valve 54 is connected, on the one hand, to the control input of the blocking valve 90 by means of a pipe 92 which starts from the pipe 83 and, on the other hand, to the working chamber 59 through a non-return valve 93 as well as through a liquid inlet bore 94 which has a throttling effect. The liquid inlet hole 94 opens into the working chamber 59 at a place where the distance from the face of the working piston

  56 in the rest position is reduced.

  In addition, the pressure chamber 48 is connected to the brakes.

  rear wheels 97, 98 via line 53, a two-way / two-position electromagnetically operated valve which is normally open, and a line 96.

  The working chamber 59 is also connected to a front wheel brake 103 by means of a housing bore 99, a pipe 100, a two-way valve / two positions with electromagnetic control 101 and a line 102. Likewise, the working chamber 70 is connected to another front wheel brake 108 by means of a housing bore 104, a line 105, a two-way valve / two electromagnetic control positions 106 and a line 107. The directionally controlled valves 101 and 106 are designed in the same form as the directionally controlled valve 95 and are also actuated by means of the monitoring circuit of the

  slip if slip occurs.

  In addition, the lines 96, 102 and 107 can be connected to the return line 84 through a normally closed two-way / two-position valve 109, 110 and, respectively, 111, and they can be actuated. by means of

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  slip monitoring circuit.

  If braking is initiated by applying an actuating force F to the brake pedal 43, the valve seat 28 is first pushed against the valve closing member 27 by means of the actuating piston 5 and the discharge valve 24 is closed, then, after a short additional stroke of the piston 20, the supply valve 22 is opened by means of the pusher 29, which allows the pressure liquid to pass from the source of pressure liquid 11 in the pressure chamber 48 via the inlet chamber 8, the supply valve 22 then open, the chamber 51, the axial bore 50 and the radial bore 49. The pressure which is then established in the pressure chamber 48 causes the displacement of the amplifier piston 5, hitherto retained by the friction force of the annular seals 9, 10 and 36, in the direction of the actuating piston 15 up to 'that the supply valve 22 is closed, so that a pressure proportional to the f braking orce F is established in the pressure chamber 48 and acts on the rear wheel brakes 97 and 98 through the two-way valve / two positions 95. Due to the displacement of the amplifier piston 5 20 to the left, the pressure liquid is evacuated from the outlet chamber 55 to the supply tank 17 through the open blocking valve 90, and the working piston 56 is moved simultaneously, the valve 66 thus being closed and the pressure being increased in the working chamber 59. The non-return valve 93 prevents the pressure liquid from being discharged from the working chamber 59 through the liquid inlet hole 94 until the liquid inlet hole 94 is covered by the annular seal 57 of the working piston 56. The increase in the pressure in the working chamber 59 not only causes the actuation of the front wheel brake 30 via the two-way valve / two open positions 101 but also the displacement of the second working piston 60 and, simultaneously, the closing of the valve 72 and an increase in the pressure in the working chamber 70, so that the pressure which acts on the front wheel brake 103 is also exerted 35 on the other front wheel brake 108 through the two-valve -1 -y'h- 2602835 ways / two open positions 106. When the brake pedal 43 is released, the pistons 5, 56 and 60 are recalled by means springs

pressure 61 and 78.

  If, upon actuation of the brake pedal 43, an excessive slip value is reached at one or more wheels of the vehicle and a tendency to lock appears, for example, on the front wheel fitted with the front wheel brake 103, the slip monitoring circuit transmits actuation signals to the two-way / two-position valves 101 and 110, which has the effect of modifying the switching state, so that the connection - between the working chamber 59 of the master cylinder 2 and the wheel brake 103 is interrupted and a connection is established between the wheel brake 103 and the common return line 84 through the valve 110 which then forms a freeway. When the valves 101, 110 15 assume this position, pressure fluid is taken from the wheel brake 103 and transmitted to the supply tank 17, which has the effect of lowering the wheel cylinder pressure in the

wheel brake 103.

  Directional control valves 95, 109 and / or 106, 111 are actuated correspondingly when the slip monitoring circuit detects a critical slip value at

level of the corresponding wheels.

  As soon as the slip monitoring circuit is triggered, the electromagnetic control main valve 54 is switched, the connection between the annular chamber 63 and the return line 84 being thus interrupted, but that between the pressure chamber 48 and the line 83 being maintained. The outlet pressure of the main valve 54 causes, on the one hand, the closing of the blocking valve 90 and, on the other hand, a pressurization of the outlet chamber 55, which then acts as a recall, through the non-return valve 93 and the liquid inlet hole 94 as long as the annular seal 57 is on the left side of the liquid inlet hole 94. The amplifier piston 5 is then returned by means of the pressure which is being established in the outlet chamber 55. At the same time, the working pistons 56

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  and 60 are recalled until the annular seal 57 again covers the liquid inlet bore 94, which has the effect of enclosing the pressure liquid in the outlet chamber 55 and preventing displacement of the amplifier piston 5. When the sliding has disappeared, the entire actuating stroke is

  available again immediately.

  Due to the supply of pressure liquid from the source 11 via the small stage 7 of the amplifier piston 5, the annular seals 9 and 10 are subjected to less radial pressure 10 only when the inlet chamber is arranged at the circumference of the large piston stage, so that a reduced friction force is manifested between the annular seals 9 and 10 and the amplifier piston, which causes a force of answer also more reduced. In this case, it is necessary for the amplifier piston 5 to move without pressure over the entire working length in order to evacuate the leak from the rear annular seal 10 towards the supply tank 17 and to guarantee the evacuation of the pressure liquid regulated by pressure. However, it is not essential to extend the amplifier of the total stroke length due to this displacement, because the

  total length of the return device is reduced. This reduction is obtained in that the working piston 56 has to travel only a partial stroke before covering the liquid inlet bore 94, so that, when a slip occurs, the liquid 25 pressure can enter the return chamber 55 and push the amplifier piston 5 into the partial stroke position.

  Another advantage of this amplifier is that it has only one external seal without pressure, namely the seal

  annular 36, the leakage of which is practically insignificant.

  2 shows a modified part of the braking system according to Figure 1 in the form of a detail, the identical parts being designated by the same reference numerals. In this variant, a second annular seal 57a is arranged at the circumference of the working piston 56 ′ instead of the non-return valve 35 93 according to FIG. 1 and has been designed so that the drilling - i3- 2602835

  liquid inlet 94 opens in an annular chamber 112 situated between the two annular seals 57 and 57a at the circumference of the working piston 56 'when the working piston 56' adopts its rest position.

  The operating mode of the braking system is not changed by replacing the non-return valve 93 with the seal

annular 57a.

  Furthermore, in the embodiment according to the figure

  2, the guide pin 58 according to. FIG. 1 has been replaced by a pin 113 having a mushroom-shaped head 114 which rests against the bottom of a blind bore 115 formed in the large stage 6 of the amplifier piston 5 and which guarantees better concentric transmission of the pressure force from amplifier piston 5 to working piston 56 '.

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Claims (7)

  1. Hydraulic force amplifier, dsti in prtialir to a motor vehicle braking system, in particular with anti-lock regulation, in which a casing houses a pressure chamber which is disposed between an amplifier piston and an actuation piston coaxial and which can be connected to a source of pressure liquid via an inlet chamber delimited by a circumferential part of the amplifier piston, the casing and two annular seals, via a hole axial communicating with the inlet chamber and via a supply valve 10, and which can be connected to a supply tank through a discharge valve, these two valves being designed in the form of seat valves arranged in the amplifier piston and operable according to the position of the actuating piston, characterized in that the amplifier piston (5) is designed as a two-stage piston do nt the small stage (7) of smaller external diameter defines the inlet chamber (8), in that the supply valve and the discharge valve (22, 24) are located one behind the other and coaxial with the axis of the pistons, and in that the pressure chamber (48) is interposed between the large piston stage (6) of larger diameter and the chamber inlet (8).   2. Amplifier according to claim 1, characterized   in that an outlet chamber (35) communicating with the supply tank (17) over the entire stroke of the piston is applied against the circumference of the small piston stage (7), at the end of the inlet chamber (8) remote from the large piston stage (6), said outlet chamber being sealed by one (10) of two annular seals (9, 10) and by a third annular seal (36) '.   3. Amplifier according to claim 1, intended for a   anti-lock regulation motor vehicle braking system in which the face of the amplifier piston (5) opposite the cham- - 15 - 2602835   pressure bre (48) rests against a coaxial working piston (56, 56 ') of a brake cylinder (2) of the braking system, this working piston (56, 56') acting on a working chamber ( 59), being mounted sealed with respect to the casing (4) and delimiting, with the casing 5 (4) and said face of the amplifier piston (5), a second outlet chamber (55) which communicates with the reservoir supply (17), characterized in that a blocking valve actuated as a function of pressure (90) is located on the connection pipe (84 to 91) leading from the second outlet chamber (55) to the ali10 tank mentation (17), and in that the pressure chamber (48) of the amplifier (1) is connected to the control input of the blocking valve (90) through a main valve actuated according to the slip ( 54) and to the second outlet chamber (55) when a part   of the working piston stroke has been covered.   4. Amplifier according to claim 3, characterized in that a liquid inlet throttling hole (94) which is made in the casing (4) and which communicates, c8té inlet, with the pressure outlet of the main valve (54) is connected, c8té outlet, to the second outlet chamber (55) after having been covered by the working piston (56, 56 ') when the partial stroke has been covered. 5. Amplifier according to claim 4, characterized   in that a non-return valve (93) is installed on the link connecting the control inlet of the blocking valve (90) and the liquid inlet hole (94).   6. 'Amplifier according to claim 4, characterized in that the liquid inlet hole (94) opens into the working chamber (112) when the working piston (56') is in the rest position, said chamber annular being delimited by the working piston (56 '), by the casing (4) and by two annular seals   (57, 57 ') arranged between the working piston (56'-) and the casing (4).   7. Amplifier according to any one of claims 3 to 6, in which the amplifier piston (5) rests against the working piston (56 ') by means of a coaxial pin 35 (113), characterized in that the pin (113) has a mushroom-shaped head (114).