GB2098685A

GB2098685A - Breakaway protection device for a trailer brake system

Info

Publication number: GB2098685A
Application number: GB8211514A
Authority: GB
Original assignee: Mannesmann Rexroth AG
Current assignee: Bosch Rexroth AG
Priority date: 1981-05-05
Filing date: 1982-04-21
Publication date: 1982-11-24
Also published as: GB2098685B; DE3117729C2; FR2507985A2; DE3117729A1; ATA169982A

Abstract

The device includes a valve (6) biased (to position B) by spring (9) to close a trailer brake supply line (3) and connect an accumulator (7) to trailer brakes (5) on breakaway, the valve being normally retained by a locking device (8) against the bias (position A as shown), with the supply line (3) connected to the brakes and, via check valve (13), to the accumulator: the locking device (8) is disengaged on loss of pressure in the brake line on breakaway at connection (2) to allow the valve to move to position B. The locking device is responsive even to loss of residual pressure in line (3) when brakes (5) are not actuated. Valves (17,18) provide the residual pressure. The locking device (8) is preferably reset on reconnection at (2) in response to pressure in line (3), and may be actuated manually for testing by a pin (734', fig. 4 not shown). <IMAGE>

Description

SPECIFICATION Breakaway protection device for a trailer brake system The invention relates to a breakaway protection device for a trailer brake system.

A device of the kind proposed in German Specification No. 30 05 703 (GB Specification 2069645) operates reliably with very little attention, due to the provision of a pressure accumulator or reservoir which forms part of the trailer brake system, is adapted to be charged up by the brake pressure and always provides the necessary energy for braking without any kind of servicing. Since the accumulator supplies the normal brake lines when it operates, additional braking facilities can be omitted. In the event of the trailer braking away from the drive vehicle, the accumulator is adapted to be connected automatically and directly to the brake lines with closure of the pressure line. Accordingly, the moving member is spring-biased towards its operative position and is retained in its normal position by a retaining pin.The same is secured to a ripcord which in the event of a breakaway disengages the pin from the moving member so that the same can move into its operative position. In this proposed breakaway protection device, the ripcord must be coupled with the trailer by way of the retaining pin separately from the coupling of the trailer to the draft vehicle; consequently, there is a risk with this known device that, for instance, because of the driver's negligence, the ripcord may fail to be connected after the trailer has been coupled with the draft vehicle, in which event the trailer cannot be braked automatically in the event of a breakaway.Another disadvantage of this known system is that no indication is given of damage to the trailer brake system, e.g. rupturing of a brake hose or brake pipe, with detriment to the safety of the complete system - i.e., the towing vehicle and the towed vehicle. Since as a rule it is only in emergencies - which are relatively rare occurrences that the moving member moves into its operative position - i.e., the protection device is required to operate fairly infrequently - another risk associated with this known protection device is that either the ripcord catches in its guide and/or the retaining pin seizes up in its guide due to corrosion. Consequently, in the event of a breakaway the ripcord is pulled, but fails to move the moving member out of its inoperative position.The protection device therefore becomes increasingly unreliable in the course of its working life and requires continuous servicing and attention.

It is therefore the object of the invention so to develop the breakaway protection device in accordance with German Specification 3005703 that the unreliable ripcord can be omitted and the trailer can be braked reliably even though there may be disturbances in the trailer brake system, such as a leak therein, the device always operating reliably and therefore increasing safety.

The present invention provides a breakaway protection device for a trailer brake system whose brake lines can be supplied by the draft vehicle through a pressure line, the device comprising a pressure accumulator or reservoir which can be charged up from the pressure line through a check valve and which, in the event of the trailer breaking away from the draftvehicle, is automatically connected directly to the brake lines with closure of the pressure line, the device also comprising a directional valve having a sliding member which is retained in a normal position against the biasing of a spring and which can be pressed by spring force into an operative position in which it keeps the pressure line closed and the check valve of the pressure accumulator open towards the brake line, characterised in that a pressure-retaining valve disposed in the trailer brake circuit before the pressure line enables a residual pressure to build up while the trailer brakes are 'off'; and the sliding member is retained in its normal position against its spring biasing by means of a snap-action locking device disenageable by the residual pressure when the pressure in the pressure line drops.

The effect of arranging the pressure-retaining valve atthe place in accordance with the invention is that after operation of the trailer brakes, the return springs thereof build up in the pressure line a residual pressure which can be adjusted fairly accurately and which opposes the check valve communicating with the draft vehicle. In accordance with the invention, the residual pressure co-operates with a snap-action locking device which at residual pressures above a predetermined level locks the moving member in its normal position. When the pressure in the pressure line drops below the predetermined residual pressure as a result of a breakaway, the snap-action locking device releases the moving member, the same then being urged by its biasing springs into its operative position.These features in accordance with the invention make a ripcord and a retaining pin unnecessary and ensure that the step of coupling the trailer to the draft vehicle automatically readies the breakaway protection device for operation. The residual pressure may drop below its critical level because of a leak in the pressure line. In this case the trailer brakes could not reliably be supplied at the braking pressure of the draft vehicle.

However, the breakaway protection device in accordance with the invention operates independently in such a case and brakes the trailer, the pressure line connection being separated from the draft vehicle.

Consequently, the safety of the draft vehicle is always ensured in the event of a hose rupture since, in contrast to the earlier solution of the problem, the breakaway protection device in accordance with the invention responds to faults in the hose system.

Also, in accordance with the invention all those parts of the breakaway protection device which used to be exposed to the environment and therefore to corrosion can be omitted, and so the components which are important for operation of the protection device cannot be corroded. The device therefore always operates reliably for a prolonged period and without any kind of attention.

The snap-action locking device in accordance with the invention has the particular further advantage thatthe moving member-i.e., the main piston-can move into its operative position and its inoperative postion in defined and very brief operating times.

The snap-action locking device ensures that the spring rate of the actuating spring is designed for optimum operating times- i.e., the energy stored in the biasing springs can be very considerable, yet even at low line pressures a direct communication with the brakes is maintained. If, however, the pressure in the pressure line drops below a safety level, the full energy of the spring biasing is released abruptly so that the valve seat can be closed very rapidly. The snap-action locking device also ensures that the piston takes up clearly defined end positions, to the exclusion of positions caused by jamming.

Conveniently, the check valves are biased with different spring forces for instance, the check valve communicating with the trailer brake circuit can be used at a very low pressure to ensure that the trailer brake circuit is supplied at a very reduced backpressure.

Preferably the sliding member is a main piston which is guided in a chamber of a casing and whose front end is a member for closing a pressure line connection, the same extending into the latter chamber coaxially thereof, the generated surface of the piston being a land for controlling radial actuation of the check valve of the pressure accumulator, the latter valve extending radially into the chamber by way of a connection for the accumulator, the brake line communicating freely, with the piston both in its normal position and in its operative position, with the chamber; and when in its normal position the piston bears on the chamber by way of at least one shaped member of the snap-action locking device.This is a very advantageous form of the invention; the mechanical valve-moving facility is disposed in a closed casing chamber at a place always immersed in hydraulic fluid; consequently, the guide surface between the moving member and the casing cannot corrode, with the result that the reliability of the trailer brake system is increased appreciably. Another advantage ofthis feature is that the various actuating and control elements can be received in a very small space, thus greatly simplifying construction of the protection device and therefore making its operation more reliable.

In a preferred embodiment of the snap-action locking device the or each shaped member is actuable by a control piston movable against an adjustable force by the residual pressure into an end position in which it force the shaped members into a snaplocking contact between the main piston and a casing recess.

Devising the snap-action locking device in this way leads to the advantage that the shaped members are actuated positively, so that the movement pattern of the shaped member can be controlled accurately.

The control piston can be guided separateiy from the main piston in a separate casing bore (cf. subclaims 6 to 8) or in a bore in the main piston (cf. sub-claims 9to 11).

Constructing the snap-action locking device with the features described in sub-claims 6 to 11 below provides the desirable feature that the locking mem bers-i.e.,the shaped members-are always immersed in hydraulic fluid and are therefore always ready to operate. In the embodiments described in subclaims 6 to 11, control piston position can be controlled readily and accurately by means of a pressure-holding spring which maintains the control piston in equilibrium against a pressure which correspondstothe residual pressure and which is operative on the other end face of the control piston.

The construction in accordance with subclaim 8 provides the advantage that the annular grooves in the control piston and in the main piston can be similar.

The construction disclosed by subclaim 9 leads to the further advantage of more reliable operation of the snap-action locking device, combined with simplification of casing construction - only one bore is needed in the casing, just to receive the main piston since a number of shaped members distributed uniformly around the periphery of the main piston transmit the retaining force radially, and therefore with uniform distribution, to the main piston so that no tilting moment can actthereon and the breakaway protection device cannot jam.

The embodiment disclosed by subclaim 16 has the advantage of enabling the control piston and the main piston to be constructed as one operating unit, thus reducing the number of moving parts. The advantages of the embodiments hereinbefore outlined remain and the further advantage is provided of a considerable reduction in the costs of the breakaway protection device due to the considerably greater simplicity of construction.

If the breakaway protection device is in accordance with subclaim 23, the moving member or the control piston is retained in its operative position in a state of snap-action locking, thus ensuring that the control piston can return to its normal position only when the pressure in the pressure line is greater than the break-operating pressure. This feature ensures that when the trailer is recoupled with the draft vehicle, the trailer brakes remain "on" until the draft vehicle brake system builds up sufficient pressure to return the control piston reliably to its normal position.

Since that end face of the main piston and of the control piston which is remote from the piston head is at atmospheric pressure, very simple constructional means can be used to achiece manual operation of the breakaway protection device and, therefore, reliable securing of the trailer by a simple manual intervention by a release pin, connected either two the biasing means orto the control piston, extending to the outside through the casing wall. The breakaway protection device can then be reliably released both mechanically and hydraulically, thus considerably expanding the possible range of uses of such device.

Other advantageous embodiments of the invention are disclosed by the other subclaims.

Embodiments of the invention will be described in greater detail hereinafter with reference to diagrammatic drawings wherein: Fig. lisa theoretical diagram of a circuit arrangement of a breakaway protection device.

Fig. 2 is a section through a first embodiment of such device; Fig. 3 is a section through a second embodiment of the device, and Fig. 4 is a section through a third embodiment of the device.

In Fig. 1 there can be seen a trailer brake circuit 1 comprising: a pressure line 3 connected by way of a trailer coupling or quick-acting coupling 2; and a brake line 4 connected to brake cylinders 5 of the trailer. A breakaway protection device 10 is provided between the lines 3 and 4 and comprises a directional valve 6, a pressure accumulator or reservoir 7 and a moving system having an abutment 8 for the valve 6, a moving member 16 and a compression spring 9 acting thereon.

The trailer brakes each have a return spring 5a, while a system embodied bytwo antiparallelconnected check valves 17, 18 is disposed in the trailer brake circuit 1 before the pressure line 3. Valve 18 opens towards trailer brake circuit 1 and experiences a very reduced biasing pressure (the threshold pressure to be overcome in this direction can be substantially atmospheric pressure (A p = 0 bar), while check valve 17 is adapted to open towards the draft vehicle and is biased by a spring 17a which is overcome at a predetermined pressure of e.g. 5 bars (Apt5bars).

Valve 6 has two positions. In its first or normal operative position A it places pressure line 3 in communication with brake line 4. The same comprises a branch 11 from which a charging line 12 for reservoir 7 branches off. Charging line 12 extends by way of the directional valve 6, reservoir 7 charging up by way of a check valve 13 when valve 6 is in its normal position A. Valve 6 is retained in its position A by the biasing of the spring 9 and by bearing on the abutment 8. When valve 6 is in position A, shown in Fig. 1, and the brake cylinders are "off", the return spring 5a of the brakes 5 forces hydraulic fluid through check valve 17 into the draft vehicle circuit, so that a residual pressure of at least 5 bars remains operative in the pressure line 3.

With valve 6 in its second position B, there is no communication between lines 3 and 4 and reservoir 7 communicates directly with brake line 4.

Acontrol line 19 branchesoffthe pressure line3 after the check valves 17, 18 and extends to a device 70 for actuating the abutment 8.

The device 70 maintains the abutment 8 in operative engagement with the moving member 16. In response to a breakaway of the trailer from the draft vehicle-in which eventthe coupling 2 first releases from the draftvehicle,whereafterthe pressure line 3, which is still connected to the draft vehicle, is torn away - or in response to a pressure drop in the pressure line 3, abutment 8 disenages from member 16, the valve 6 simultaneously being moved by spring 9 into its second end or operative position B in which the brake cylinders 5 communicate directly with the reservoir 7, the trailer therefore experiencing an emergency brake application and the pressure line 3 simultaneously being closed.

Fig. 2 shows an embodiment of the protection facility of Fig. 1. As can be gathered from Fig. 1,the breakaway protection device 10 has a connection to the pressure line 3, brake line4 and charging line 12.

The device 10, which is received in a casing 20, has a casing chamber 21 with, coaxial thereof, a casing bore; a connector 23 having a spigot or the like 24 for the pressure line is received, preferably by being screwed in, in a front portion 21a of chamber 21.

Also extending into portion 21a is a radial brake line connection 25. Casing 20 also comprises a reservoir connection 26 for check valve 13, connection 26 extending into a rear portion 21b of chamber 21.

Connection 26 widens by way of a valve seat 27 to form a check valve bore 28 receiving a check valve spring 29 which, bearing on casing 20, for instance, by way of an axial securing ring, presses a valve-ball 31 on the valve seat 27.

Disposed in chamber 21 after the connections 24 and 25 is the main actuating piston - i.e., the sliding member 16. In its part near the reservoir connection 26 the piston 16 is T-shaped and comprises a piston head 61 and stem 62, the latter being adapted to bear on rear wall 22 of casing 21. A helical compression spring 9 is received between rear face 68 of head 61 and rear wall 22 of casing 20 and biases piston 16 downwardly in Fig. 2. The same is retained in the position shown by an abutment (or a retaining ball) 8 which bears in a radial bore 30 in casing 20; the position shown corresponds to the normal or inoperative position A of the main piston 16 of Fig. 1.

That end face of piston 16 which is distal from the head 61 forms a sealing-tight boundary of the chamberforthe spring 9, the latter chamber communicating with atmosphere by way of a communicating duct 221 shown in chain lines.

The piston head 61 takes the form of a stepped cylinder having a front cylindrical portion 63, a rear cylindrical guiding portion 64 and, disposed between the portions 63 and 64, a conical control portion 65. Front portion 63 is stepped radially inwardly in relation to the casing chamber 21. At its front end the piston 16 has a sealing head having on its front face a sealing spherical cap 66. With the piston 16 in the normal position A shown, the front portion 63 is disposed centrally below the reservoir connection 26 and the cap 66 is at an axial distance D from the pressure line connection 24.

Disposed in the reservoir connection 26 is a valve ball 67 of smaller diameter than the reservoir connection 26. Front part 63 of piston head 61 maintains ball 67 in the reservoir connection 26, some radial clearance remaining between ball 67 and ball 31 of check valve 13.

With the device 10 in the normal position shown, pressure line connection 24 communicates with brake line connection 25 and, via a bridging bore 69, with reservoir connection 26, so that the pressure in brake line 4 can charge up reservoir 7.

Between the sealing head 61 and the rear head 61 the piston 16 has a guide portion 160 formed with an annular groove 161 which, with piston 16 in the position shown, registers exactly with radial bore 30. Ball 8 enagages in groove 161 and retains piston 16 in the position shown against the force of spring 9. Ball 8 is in abutting engagement with another shaped member-a ball 71 in Fig. 2 which bears on a cylindrical control surface 72 of a control piston 73 guided sealingly in a cylindrical bore 21' in casing 20.

Piston 73 has two guide portions 74,75, two terminal abutment portions 76,77 and a snap-locking portion 78 in the form of an annular groove between the portions 74 and 75. The abutment portion 77 is near the casing wall 22 and has extending around it a helical compression spring 79 bearing at one end on casing 20 and at the other end (the top end in Fig. 2) on the guide portion 75. The bottom end of piston 73 - i.e., its end face distal from the spring 79 -experi- ences by way of a control pressure line connection 191 the pressure in the control line, the latter pressure corresponding to the pressure in the pressure line 3. The position in which piston 73 is shown is its normal end position since the pressure in the pressure line 3 is greater than the pressure applied by spring 79 to piston 73.Spring 79 is such that its force is overcome when the pressure in the pressure line 3 is greater than the residual pressure determined by the check valve 17. Consequently, by way of the position of the control piston 73 and of the snap-action system comprising surface 72, shaped member 71, ball 8 and annular groove 161, the pressure in the pressure line 3 maintains the moving member-i.e., the main piston 16 - in the normal position A. That chamber ofthe bore 21' which receives the compression spring 79 communicates byway of a duct 221 with atmosphere.

If for any reason the pressure in the pressure line 3 drops below the residual pressure, spring 79 moves piston 73 into its bottom end or abutment position so that annular groove 78 is moved into a position opposite the radial bore 30. Depth T78 of groove 78 is substantially the same as the depth T161 of groove 161; consequently, because of the action of the spring 9, the piston 16 can, by moving the ball 8, move the member 71 into the annular groove 78, the member71 descending in Fig. 2. Consequently, the piston 16 descends (in Fig. 2), travelling through an axial distance D, into an operative position B in which it bears by way of its cap 66 on a sealing seat 24a of portion 23 of the pressure line connection 24 and closes the same.The biasing of the spring 9 is such that it is still biased when the piston 16 is in the operative position B. During this movement, due to the movement of control portion 65 of piston 16 and after the radial clearance between the balls 67 and 31 has been taken up, communication is established by way of the bridging bore 69 between the reservoir 7 and the brake line 4, the ball 67 disengaging the ball 31 from the valve seat 27.

After the trailer has been recoupled with the draft vehicle, the cap 66 can be disengaged from the valve seat 24a by operation of the brakes. The piston 16 therefore rises and the groove 161 returns to its position shown in Fig. 2, so that the pressure acting on the end face of the control piston 73 returns the same, by moving the members 71,8. into its inoperative position. The snap-action locking mechanism is therefore operative again.

Fig. 3 shows another embodiment of a breakaway protection device wherein the main piston 16 and the control piston 73 are combined to form a single piston. In this embodiment like elements have the same references as in Fig. 2. The reservoir connection 26 for the check valve 13 extends to the front portion 21a of casing chamber 21 at a place diametrically opposite the brake line connection 25. The moving member 16 is disposed after the connections 24-26 and is in the form of a cylindrical member having a piston head 61 and a piston guide stem 62'. The same is formed with a recess 621 engaged by the compression spring 9 which bears on rear wall 22 of casing 20. Front portion 63 of the member or piston 16 bears the cap 66. Head 61 is basically similar to the head 61 shown in Fig. 2.Piston 16 is formed between head 61 and stem 62 with a diametric bore 622 receiving a compression spring 82 and two shaped members 81 biased thereby against one another. With the piston 16 in its inoperative position A shown in Fig. 3, the members 81 engage in an annular groove 202 of casing 20.

Due to the pressure in the pressure line 3, the members 81 maintain the piston 16 in the snap-locked position shown against the pressure of the spring 9.

If for any reason the pressure in the pressure line 3 drops below the residual pressure determined by the spring 17, spring 9 moves piston 16 into its operative position B by pressing the members 81 into the bore 622 against the force of the biasing spring 82. In this movement the check valve 13 opens as hereinbefore described and communicates with the brake line 4, for when the piston 16 is in its operative position B, there remains between the front portion 63 or control portion 65 of piston head 61, on the one hand, and the wall of the chamber 21, on the other hand, an annular gap via which hydraulic fluid can flow from reservoir 7 to the brake cylinders.

After the trailer has been recoupled with the draft vehicle, the cap 66 can be disengaged from the valve seat 24a by operation of the brakes, as hereinbefore described. The piston 16 therefore moves to the right (in Fig. 3) and the members 81 snap back into the groove 622. Recess 68a in piston 16 communicates via a bore 201 in casing 20 with atmosphere so that no backpressure can build up on the back of piston 16.

If the bore 20 is formed with another annular groove or recess (not shown) spaced apart from the groove 202 by a distance D in the closing direction, the piston 16 takes up a second snap position in its operative position B, thus ensuring that the piston 16 can be returned to its normal position A only when the pressure built up by the draft vehicle brake system is greater than the normal trailer brake operating pressure. Once such a pressure has built up after reconnection of the trailer, the piston snaps back into its normal position A, and so there is no risk of the piston 16 taking up a floating position between its two positions A and B.

Fig. 4 shows another embodiment of the breakaway protection device, this embodiment mainly being a combination of the embodiments of Figs. 2 and 3 with the control piston 73 of Fig. 1 integrated into the main piston 16 of Fig. 3. Like elements have the same reference numerals in Fig. 4 as in Figs. 2 and 3 but with the addition of an apostrophe.

Main piston 16' receives control piston 73' which is guided sealingly and for sliding movement in an inner bore 611' by way of guide portions 74' and with the inter-position of seals 731 732'. With the device in the position shown, the pressure line 3 is open and the control piston 73' is in its rear end position, bearing by way of an abutment 77' on the base of inner bore 611' of main piston 16'; in its rear end position the piston 73' acts by way of its cylindrical surface to press into annular groove 202' the shaped members 81', the same being guided in radial bores 612'. Consequently, the main piston 16' takes up its normal or inoperative position in which it bears by way of the members 81' and against the force of the compression spring 9' on the casing 10.The control piston 73' is maintained in the end position shown by the pressure in the pressure line 3 and againstthe force of a biasing spring 79' which bears on the rear guide portion 732' of piston 73' and on the base of inner bore 611'. Behind control face 72' the piston 73' is formed with a recess, e.g. an annular groove 78', in which the inner shaped member 81' can engage to release the snap-action sealing contact of the members 81' with the casing 10. The rear guide portion 732' is connected to a release pin 734' which extends to the exterior through chamber621 receiving spring 9', and through casing 10.

When pin 734' is operated or when the pressure in the pressure line 3 drops belowthe residual pressure, the pressure acting on the end of piston 73' ceases to be able to maintain piston 73' in its inoperative position (shown in Fig. 4) against the force of the spring 79'; consequently, piston 73' moves to the left (in Fig. 4) and allows the members 81' to move radially towards piston 73', releasing as they do so the main piston 16' which the spring 9' can then move into its closed or operative position to close the connection 23. The piston 73' is then maintained in its front end position by positive engage ment, by way of the members 81', with the groove 78' and with the main piston 16'.

This embodiment can have a front stop for the piston 73'. Also, a snap-action facility (cf. Fig. 3 when the member 81 engages in a recess) can come into operation when the main piston 16 is in its operative position, so that in the embodiment shown in Fig. 4 as in the embodiment shown in Fig. 3 the main piston 16' can be returned only when the pressure acting on the end of piston 16' is greater than the normal trailer brake operating pressure, thus ensuring that the piston 16' makes its return movement very fast and is moved reliably into its normal position.

Claims

1. A breakaway protection device for a trailer brake system whose brake lines can be supplied by the draft vehicle through a pressure line, the device comprising a pressure accumulator or reservoir which can be charged up from the pressure line through a check valve and which, in the event of the trailer breaking away from the draft vehicle, is automatically connected directly to the brake lines with closure of the pressure line, the device also comprising a directional valve having a sliding member which is retained in a normal position against the biasing of a spring and which can be pressed by spring force into an operative position in which it keeps the pressure line closed and the check valve of the pressure accumulator open towards the brake line, characterised in that a pressure-retaining valve disposed in the trailer brake circuit before the pressure line enables a residual pressure to build up while the trailer brakes are 'off'; and the sliding member is retained in its normal position against its spring biasing by means of a snap-action locking device disengageable by the residual pressure when the pressure in the pressure line drops.

2. A device according to Claim 1, characterised in that the pressure-retaining valve is embodied by two antiparallel-connected check valves so connected to the trailer half of the coupling as not to be able to be torn away.

3. A device according to Claim 1 or Claim 2, characterised in that the check valve controlling the trailer brake circuit experiences a lower biasing force than the check valve extending towards the draft vehicle.

4. A device according to any preceding Claim characterised in that the sliding member is a main piston which is guided in a chamber of a casing and whose front end is a memberforclosing a pressure line connection, the same extending into the latter chamber coaxiallythereof, the generated surface of the piston being a land for controlling radial actuation of the check valve of the pressure accumulator, the latter valve extending radially into the chamber by way of a connection for the accumulator, the brake line communicating freely, with the piston both in its normal position and in its operative posi tion,withthechamber; and when in its normal position the piston bears on the chamber by way of at least one shaped member of the snap-action locking device.

5. A device according to Claim 4 characterised in that the or each shaped member is actuable by a control piston movable against an adjustable force by the residual pressure into an end position in which it forces the shaped members into a snaplocking contact between the main piston and a casing recess.

6. A device according to Claim 5, characterised in that the main piston is formed with an annular groove in which the shaped member can engage, the same bearing in a radial bore in the chamber and being movable buy a land of the control piston.

7. A device according to Claim 5 or Claim 6, characterised in that the control piston is guided sealingly and slidingly in a cylindrical bore of the casing and experiences the force of a compression spring on one end face and the pressure in the pressure line on its other end face.

8. A device according to any one of Claims 5 to 7, characterised in that the control piston is formed with an annular groove whose depth corresponds substantially to the depth of the groove in the main piston and which, upon the control piston moving in response to a drop in the pressure in the pressure line, is moved by the residual pressure into a second end position opposite the radial bore.

9. A device according to Claim 5, characterised in thatthe main piston is formed with a number of radial bores each receiving at least one shaped member movable into and lockable in a snap-locking engagement in an annular groove in the casing by a land of the control piston, the same being guided sealingly and slidingly in the main piston coaxially thereof.

10. A device according to Claim 9, characterised in that the control piston is formed with an annular groove whose depth corresponds substantially to the depth of the annular groove in the casing and which, upon the control piston moving in response to a pressure drop in the pressure line, is moved by the residual pressure into a second end position opposite the radial bores.

11. A device according to any one of Claims 5 to 10, characterised in that that face of the control piston which is near a compression spring providing said adjustable force communicates with atmospheric pressure.

12. Adevice according to any one of Claims 5 to 11, characterised in that a spring providing said adjustable force is adapted to the biasing of the check valve communicating with the draft vehicle.

13. A device according to Claims 5 to 12, characterised in that springs providing said adjustable force can be overcome at a pressure residual of e.g., 5bars.

14. A device according to any one of Claims 5 to 13, characterised in that the main piston has a stem which, with the main piston in its normal position, abuts a casing rear wall, the head of the main piston being movable in its chamber with clearance.

15. A device according to any one of Claims 5 to 13, characterised in that the main piston has a guide stem which is guided sealingly in the bore of the casing chamber and is formed with a coaxial recess receiving a helical compression spring bearing on the casing rear wall.

16. A device according to Claim 4, characterised in that the main piston is formed with at least one radial bore receiving at least one shaped member borne by way of a spring device and engaging, with the main piston in its normal position, in an annular groove in the casing chamber in which the main piston is located.

17. A device according to Claim 16, characterised in that the main piston has a guide stem which is guided sealingly in the bore in the casing chamber, the head ofthe main piston being movable with clearance in such chamber.

18. A device according to Claim 16 or Claim 17, characterised in that the guide stem is formed with a coaxial recess receiving a helical compression spring which bears on the casing rear wall.

19. A device according to Claim 18, characterised in that the main piston head is in the shape of a stepped cylindrical member having: a cylindrical front portion which forms the seat member, carries a part-spherical sealing member and is stepped radially inwardly from the wall of the casing chamber; a cylindrical rear guiding portion guided in the chamber; and, disposed between the front por tion and the rear portion, an intermediate portion serving as the land of the main piston.

20. A device according to Claim 19, characterised in that a ball for opening the check valve is disposed in the pressure accumulator connection between the check valve and the main piston head and, with the main piston in its normal position, bears on the front portion of the head.

21. A device according to Claim 20, characterised in the ball for opening the check valve, with the piston in its operative position, bears on the said intermediate portion of the main piston head.

22. A device according to any preceding Claim characterised in that a ball of the check valve closes the pressure accumulator connection, said ball being urgeable on to a valve seat of the bore of the check valve buy a check valve spring disposed inthe latter bore, which is coaxial with the last-mentioned connection, and bearing on the casing.

23. Adevice according to Claim 9 characterised in that at least one of the shaped members in the radial bores of the main piston can, with the piston in its operative position, be snapped into another recess in the casing bore.

24. A device according to Claims 4 and 23, characterised in that the shaped members in the snap-action locking device and in the radial bores of the main piston have spherical surfaces.

25. A device according to any preceding Claim characterised in that a helical compression spring balances the sliding member against a pressure of e.g. 70 bars acting on its end face.

26. A device according to any preceding Claim, characterised in that after recoupling of the trailer to the draft vehicle the piston is automatically restored, by the brake line pressure acting on it, to its normal position, against the force of its spring bias, in which position the snap-action locking device is operative again.

27. A device according to any preceding Claim characterised in that the snap-action locking device can be unlocked manually by means of a release pin which extends through the casing wall.