GB2399609A

GB2399609A - Brake fluid circulation

Info

Publication number: GB2399609A
Application number: GB0412169A
Authority: GB
Inventors: Richard Arnold Bass
Original assignee: AP Racing Ltd
Current assignee: AP Racing Ltd
Priority date: 2000-03-30
Filing date: 2001-03-28
Publication date: 2004-09-22
Anticipated expiration: 2021-03-28
Also published as: GB2399609B; GB0412169D0

Abstract

A brake circuit has brake means (236; 336) connected with a master cylinder (311) via a brake application line (235) and a brake return line (237). Non-return valve means are arranged such that when the brake master cylinder is actuated to apply the brakes, a quantity of brake fluid is delivered to the brake means via the brake application line and that when the brake master cylinder is released, a quantity of brake fluid is drawn from the brake means via the brake return line. The brake means include at least one brake (250) having two or more brake actuators (254, 255) connected in parallel and further non-return valve means (258) are provided to prevent or reduce circulation of the brake fluid between the parallel connected actuators of the brake.

Description

BROKE FLUID CIRCULATION

This invention relates to arrangements for circulating brake fluid within a brake circuit to prevent vaporization and promote cooling of the fluid and associated brake actuator.

Such arrangements are already known. For example, US patents 5310252 and 5350223 disclose brake circuits for disc brakes which circuits include non-return valves to promote circulation of the brake fluid.

Furthermore, the applicant's co-pending patent application published as GB 2376727 describes and claims a brake circuit comprising brake means connected with a master r r cylinder via a brake application line and a brake return line, the brake master cylinder 0 having a first chamber in which brake fluid is pressurised on movement of a first piston in a brake applying direction by an operating means, the first chamber being connected via a.,, first port and a non-return valve with the brake application line of the circuit, and a,; , second chamber in communication with the brake return line of the circuit via a second port and having a second piston moveable by the brake operating means in brake applying is and releasing directions, the second chamber also being in communication with a À reservoir via a third port when the second piston is in a fully released position, the second chamber being arranged to draw-in brake fluid from the brake return line on movement of the second piston in a brake releasing direction by the operating means and establishing a connection with the associated reservoir via the third port to allow escape of aerated fluid into the reservoir when the second piston is in the fully released position.

Such a brake circuit sets up a circulation of brake fluid into and out of the master cylinder since on each application and release of the brake means a small volume of fluid is displaced from the first chamber into the brake application line and a small volume drawn out of the brake return line into the second chamber.

This circulation of brake fluid assists in the cooling of the brake fluid and brake means which is particularly important in high performance brakes where brake fluid in closed circuit brakes which do not have such a circulation of fluid can be raised to high temperatures which can result in the boiling/vaporisation of the brake fluid and subsequent brake failure.

However, the applicant has found that when a brake having two or more brake actuators connected in parallel is used in a brake circuit whiel1 circulates brake fluid, such as those described and claimed in the applicant's referenced co-pending application or those described in US patents 53 l 0252 and 5350223, there can be a tendency for brake fluid to circulate within the brake. This reduces the overall efficiency of the circulating brake fluid I o circuit.

It is an object of the present invention to provide an improved brake circuit for circulating brake fluid which overcomes or at least mitigates this problem. It is a further object to provide an improved brake for use in such a circuit. , À, Thus, in accordance with a first aspect of the invention, there is provided a brake circuit do,, comprising brake means connected with a master cylinder via a brake application line and ' . a brake return line, the bralce circuit comprising non-return valve means arranged such.,:, that when the brake master cylinder is actuated to apply the brake means, a quantity of brake fluid is delivered to the brake means via the brake application line and that when the brake master cylinder is released, a quantity of brake fluid is drawn from the brake means via the brake return line, the brake means comprising at least one brake having two or more brake actuators comected in parallel, characterised in that further non-return valve means are provided to prevent or reduce circulation of the bralce fluid between the parallel connected actuators of the brake.

The arrangement according to the first aspect of the invention improves the overall 2s efficiency of the brake fluid circulation about the brake circuit by preventing, or at least reducing, circulation of the brake fluid within the brake itself. This results in improved cooling.

The at least one brake may be a disc brake having a brake caliper, the caliper comprising a body having at least two piston and cylinder assemblies, the caliper further comprising a fluid inlet and a fluid outlet, the inlet and outlet being connected in parallel by two fluid passages, one passage supplying fluid to at least one of the piston and cylinder assemblies, s the other passage supplying fluid to at least one other piston and cylinder assembly, in which a non-return valve is provided in one of the passageways. A non-return valve may be provided in each fluid passage.

The caliper body may comprise two limbs, each limb having at least one piston and cylinder assembly, one of the fluid passages supplying fluid to the at least one piston and l o cylinder assembly in one of the limbs, the other passage supplying fluid to the at least one piston and cylinder assembly in the other limb.

Where the inlet and outlet are provided in one of the limbs, the fluid passageway which. : . supplies fluid to the or each piston assembly of the other limb may comprise a pipe which extends externally of the body of the caliper. In such an arrangement, a non-return valve., . may be provided between the inlet and a union to which one end of the external pipe is connected.

The at least one brake may be a drum brake comprising two or more brake cylinders connected in parallel.

In accordance with a second aspect of the invention, there is provided a brake for use in a brake circuit in accordance with the first aspect, the brake comprising two or more actuators connected in parallel and nonretrurn valve means arranged to prevent or at least reduce circulation of brake fluid within the brake.

The brake may comprise a disc brake caliper having at least two piston and cylinder assemblies, the caliper further comprising a fluid inlet and a fluid outlet, the inlet and outlet being connected in parallel by two fluid passages, one passage supplying fluid to at least one of the piston and cylinder assemblies, the other passage supplying fluid to at least one other piston and cylinder assembly, in which a non-return valve is provided in one of the passageways. A non-return valve may be provided in each fluid passage.

The caliper may include a body having two limbs, each limb having at least one piston and cylinder assembly, one of the fluid passages supplying fluid to the at least one piston and cylinder assembly in one of the limbs, the other passage supplying fluid to the at least one piston and cylinder assembly in the other limb.

Where the inlet and outlet are provided in one of the limbs, and the fluid passageway which supplies fluid to the or each piston assembly of the other limb may comprise a pipe which extends externally of the body of the caliper. In such an arrangement, a non-return l o valve may be provided between the inlet and a union to which one end of the external pipe is connected.

The brake may comprise a drum brake having two or more brake cylinders connected in parallel.

Several embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: ., Figure I shows a cross section through a master cylinder suitable for use in a brake circuit in accordance with the present invention; Figure 2 shows an exploded perspective view of the master cylinder of Figure 1; Figure 3 shows a block diagram of a vehicle braking circuit employing two master cylinders in accordance with F igure 1; Figure 4 shows a standard tandem master cylinder modified for use in a brake circuit in accordance with the present invention; Figure S shows a further brake circuit in accordance with the invention; Figure 6 is a perspective view of the brake circuit of Figure 5; Figure 7 shows a cross sectional through a brake caliper for use with the brake circuits of Figure I or Figure 5 in accordance with the present invention; s Figure 8 is a view similar to that of Figure 7 showing a modified brake caliper; Figure 9 is a detai led view in an enlarged scale of a non-return valve used in the brake caliper of Figure 8.

Referring to Figures 1 and 2, a brake master cylinder 10 has a body 11 at one end of, which a first chamber 12 is defined by a first piston 13 slideable in bore I la. First. , lo chamber 12 is connected with an associated brake circuit (shown in Figure 3) via a first port 14 which houses a non-return valve 15 which only allows the flow of fluid out of chamber 12. '' A second chamber 16 is defined within bore 1 1 a between first piston 13 and second piston 17. This second chamber communicates with the brake circuit via a second port 18 and with a fluid reservoir 19 via a third port 20. A bleed port 1 lb housing a bleed screw 11 c also opens into second chamber 16.

A third chamber 21 is formed within bore 1 la between the second piston 17 and the other end of the body 1 1 and is sealed by a main seal 22. The third chamber also communicates with the reservoir 19 via a fourth port 23 which ensures that the third chamber 21 remains filled with hydraulic fluid at all times thus preventing the ingress of air into the master cylinder.

Both the first and second pistons 13 and 17 are displaceable to the left from the position shown in Figure 1 by an actuating rod 24 which is connected with an associated brake pedal 40 and has a part-spherical end 24a which pushes (but cannot pull) on the second piston 17. The rod is held captive within the master cylinder body by a retaining washer and circlip 26. The first piston 13 is biased to the right as viewed in Figure I by an inner coil spring 27a which extends within the piston 13 and is supported on a stabilising rod 28 and an outer tapering coil spring 27b which acts against an abutment 13a on piston 13. As shown in Figure 2 only spring 27a may be used in certain applications. Piston 13 is provided with a square section extension rod 29 which extends within a bore 30 in the second piston 17. An annular shoulder 31 on first piston 13 engages an annular abutment 32 on the front part of second piston 17 so that the two pistons can move along bore 1 la l o as a unit.

The first piston 13 has a seal 33 which seals the first chamber 12 as piston 13 is moved to the left in Figure 1 but which allows leakage of fluid round seal 33 as the first piston, moves to the right during a brake release movement. Similarly second piston 17 carries a. , seal 34 which seals second chamber 16 during a brake application movement which, when in the fully released position shown in Figure 1, allows communication between the second chamber 16 and the reservoir 19 via the third port 20. ' ' Referring to the brake circuit shown in Figure 3, this shows master cylinder 11 and a second master cylinder 111 which is identical with master cylinder 1]. Both master cylinders are actuated from a common brake pedal 40 via push rods 24 and 124 respectively. A brake balancing pedal mechanism of the form shown in UK patent GB-B 2309066 may be used to connect rods 24 and 124 with pedal 40.

The first port 14 of master cylinder 11 is connected with a brake application line 35 which is connected in parallel with front disc brakes 36. A brake return line 37 is connected at one end in parallel with brakes 36 and at the other end with second port 18 of master cylinder 11. Reservoir 19 is connected with third port 20.

In a similar fashion second master cylinder 111 has a first port 114 which is connected with a brake application line 135 which communicates in parallel with rear disc brakes 136. A brake return line 137 is connected at one end in parallel with brakes l 36 and at the other end with second port 118 of master cylinder 111. Third port 120 of master cylinder is connected with reservoir 19.

Thus the brake circuit has two master cylinders l l and 111 which operate separate front and rear brake sub-circuits. Although both front and rear brakes 36 and 136 arc described above as being disc brakes this is not essential and, for example, rear brakes 36 could be drum brakes.

The operation of master cylinder 11 in connection with front brakes 36 will now be described in detail. It is appreciated that the operation of second master cylinder 111 lo which operates rear brakes 136 is identical.

Referring to master cylinder 11, Figure 1 shows the master cylinder in the fully brake. ,, released position in which the pistons have moved to their maximum right hand position.

To apply the brakes the brake pedal 40 is depressed which moves the push rod 24 to the,, , left as indicated by arrow X. This moves both the first piston 13 and the second piston 17 to the left via the abutment between shoulder 31 and annular abutment 32.

During brake application, first chamber 12 is pressurised by the movement of first piston l 3 to the left and hence pressurised fluid is passed to brake application line 35 via non- return valve 15. Second chamber 16 is also pressurised by return line 37. This pressure cannot leak back to the reservoir l9 as seal 34 has sealed off port 20. Hence front brakes 36 are applied.

When the brake pedal 40 is released pushrod 24 moves to the right as indicated in arrow Y in Figure 1. This allows the second piston to move to the right under brake pressure in the second chamber bringing abutment 32 out of contact with shoulder 11 on the first piston 13. Thus the volume of second chamber 16 is increased and hence brake fluid is Is sucked into second chamDcr 16 from the brake return line 37 via port 18. When piston 17 reaches the Figure I position communication is established between the second chamber 16 and the reservoir 19 and fluid can then flow from chamber 16 to the reservoir. in this condition any aerated fluid tends to rise from chamber 16 into the reservoir.

When the pressure in second chamber 16 falls as the third port 20 is opened the first piston 13 is able to move to the right under the action of springs 27a and 27b to again bring shoulder 31 into contact with annular abutment 32. This movement of the first piston to the right also assists in promoting flow of aerated fluid from second chamber 16 into the reservoir 19.

Thus, as will be appreciated, during a brake application a volume of fluid is displaced into the brake actuating circuit from the first chamber 12 by the master cylinder and during l O bralce release a volume of fluid is sucked out of the brace actuating cylinder into second chamber 16. This promotes a circulation of brake fluid around the brake circuit which, assists in preventing vaporisation ol the fluid and also promotes general cooling of the, , brace itself.

Since flow of fluid Itom the bralce actuating circuit into the second chamber 16 is not via ' ' a non-return valve there is no tendency for brace fluid pressure to be trapped in the brace.

circuit hence a quick release of brakes is achieved. Also, there is a relatively unrestricted flow of fluid into second chamber 16 which again assists in venting any aerated fluid from the system.

As will be appreciated, a single master cylinder can be used to operate the front and rear brakes 36 and 136 of Figure 3. Also, instead of connecting the brakes 36 and 136 in parallel with their associated master cylinders they could be connected in series.

Whilst the brake circuit as described above uses a specially constructed master cylinder, a standard tandem master cylinder may be modified to provide a similar function. Figure 4 shows such a construction in which components which provide the same function as the components of Figure 1 have had their reference numerals increased by 200.

In the Figure 4 arrangement first and second chambers 212 and 216 respectively are separated by a fourth unpressurised chamber 250 which is connected with reservoir 19.

Normally the first and second chambers 212 and 216 are connected to front and rear braking circuits respectively to provide the normal tandem master cylinder operation.

When used in accordance with the present Invention these chambers are connected to the brake application line 35 and brake return line 37 respectively as previously described in relation to Figures 1 and 3. The operation of the dual master cylinder when connected in this manner is the same as the master cylinder previously described in relation to Figure 1 with the first and second pistons abutting each other at surfaces 251 and 252 respectively lo to provide equivalent abutments to the components 31 and 32 of Figure 1.

Figures 5 and 6 show a further brake circuit in accordance with the invention.

Components which provide the same function as the components of Figure 3 are given the same reference numeral but increased by 200.

The brake fluid circuit of Figures 5 and 6 is similar to the circuit of Figure 3 except that. ' . the master cylinders 211, 311 are connected to their respective brake means in series. . The construction and operation of the brake master cylinders 2 1 1 and 3 1 1 is the identical -, . to the brake master cylinders 11, l l l of the brake fluid circuit described above with reference to Figure 3 and so will not be described again in detail. I-Iowever, there follows a brief description of the connection of the first master cylinder 211 with the front brakes 236a, 236b. It will be appreciated that the second master cylinder 311 and the rear brakes 336a, 336b are similarly connected and will function in a similar fashion.

The first master cylinder 211 has a first chamber which is connected to an input port 241 of a first of the front brakes 236a via brake application line 235. A second chamber of the master cylinder is connected to an output port 242 of a second of the front brakes 236b by 2s the brake return line 237. The first and second front brakes are interconnected via an intermediate brake line 243 which comects an output port 244 of the first front brake 236a to an input port 245 of the second front brake 236b.

in operation, when the brakes are applied pressurised fluid will be supplied from the first chamber of the master cylinder 211 to the application line 235 and when the brakes are released fluid will be drawn out from the return line 237 into the second chamber of the master cylinder. The fluid will thus circulate from the first chamber of the master cylinder to the first front brake 236a via the application line 235, then from the first front brake 236a to the second front brake 236b via the intermediate brake line 243 and finally from the second front brake 236b to the second chamber ol' the master cylinder via the brake return line 237.

The brake circuit, therefore, provides for a circulation of the brake fluid within the brake circuit as the brakes are applied and released. During use, particularly if the vehicle is l o used in racing conditions, the brakes are regularly being applied and released and there will be a correspondingly regular movement of brake fluid about the circuit.

All the brakes shown in the brake circuit of Figures 5 and 6 are disc brakes comprising calipers of the opposed piston type. Figure 7 shows a cross section of a caliper 250 of the second front brake 236b. It will be appreciated that the calipers of all the brakes will be essentially the same' though there may be some differences in detail owing to the fact that the calipers are modified for use on the left or right hand side of the vehicle as required.

The caliper 250 comprises a body 251 having two limbs 252, 253 which in use are positioned one on either side of a brake disc. A first limb 252 has a first pair of actuators in the form of piston and cylinder assemblies 254, whilst the other limb 253 has a second pair of actuators in the fonn of opposed piston and cylinder assemblies 255. The caliper further comprises a brake fluid inlet 245 and a brake fluid outlet 242 which are connected in parallel by two brake fluid passages 256, 257. A first of the passages 256 supplies brake fluid to the piston and cylinder assemblies 254 in one of the limbs 252, whilst the other passage 257 supplies brake fluid to the piston and cylinder assemblies 255 in the other of the limbs 253.

As so far described, caliper 250 is conventional and it will be noted that the fluid passages 256, 257 form a complete fluid circuit within the caliper. It has been found that when a caliper of this type is used in a brake circuit in which means are provided to circulate the brake fluid, there is a tendency for fluid to circulate or re-circulate within the caliper itself, moving from one passage 256, 257 to the other 257, 256, rather than flowing through the brake and around the main brake circuit as the brakes are operated.

To overcome this problem in accordance with the present invention, a nonreturn valve 258 is provided in the fluid passage 256. The valve 258 cusures that brake fluid can only flow through the passage 256 in the direction of arrow A, i.e. from the inlet 245 towards the outlet 242. 1 he non-return valve 258 can be of any suitable form but preferably is of the ball and spring type.

In operation, when the brakes are applied pressurised fluid will be supplied from the first chamber of the master cylinder 211 via the application line 235, the first front brake 236a l o and the intermediate brake line 243 to the brake fluid inlet 245 of the caliper 250.

Pressurised fluid will then flow along the first brake fluid passage 256 through the non return valve 258 to the piston and cylinder assemblies 254 in the limb 252. Pressurised fluid will also flow in the direction of arrow B along the passage 257 to the piston and cylinder assemblies 255 in the other limb 253.

When the brakes are released, brake fluid will be drawn into the second chamber of the master cylinder from the brake return line 237 which is connected to the brake fluid outlet 242 of the caliper 250. The brake fluid within the first brake fluid passage 256 is constrained to move in the direction of arrow A because of the non-return valve 258 and hence will tend to move towards and out of the outlet 242 into the return line 237. The brake fluid in the passage 257 is prevented from returning throughthe inlet 245 because ofthe non- return valve which acts between the first chamber of the master cylinder 211 and the brake application line 236 and so will tend to flow in the direction of arrow B towards the outlet 242.

It is possible that when the brakes are released, the fluid in the passage 257 could flow in the opposite direction to arrow B and into the passage 256 through the non-return valve 258.

However, in order for this to happen the fluid must open the non-return valve 258 overcoming the resistance inherent in the valve. rl'hus there will be a tendency for the fluid in passage 257 to flow in the direction of arrow B towards the outlet which is the path of least resistance.

It will be understood that the brake caliper 250 described with reference to Figure 7 could also be used in the brake fluid circuit described with reference to Figure 3 in which the brake means are connected to the master cylinder in parallel.

Figure 8 shows an alternative form of opposed piston caliper 350 that can be used wit the brake fluid circuits of 1; igure 3 or Figures 5 and 6. The caliper 350 is similar to the caliper 250 shown in Figure 7 and the same reference numerals but increased by 100 are used to identify components which perform the same function as those in Figure 7.

Caliper 350 comprises a body 351 having two limbs 352, 353, each of the limbs having three piston and cylinder assemblies 354, 355 respectively.

0 A brake fluid inlet 345 and a brake fluid outlet 342 are connected in parallel by two brake fluid passages 356, 357. A first of the passages 356 supplies brake fluid to the piston and cylinder assemblies 354 in the limb 352. A second passage 357 supplies brake fluid to the piston and cylinder assemblies 355 in the other limb 353. ; In this case, the brake fluid passage 357 is formed in part by two external pipes 357a, 357b which interconnect fluid galleries formed in either limb 352, 353 in a manner known in the art. The pipes 357a, 357b being connected to the fluid galleries by means of threaded unions 362.

A non-return valve 358 is provided in the passage 357 to ensure that brake fluid can only flow through the passage 357 in the direction of arrow B. The operation of the caliper 350 is much the same as for the caliper 250 described above, except that the non-return valve 358 controls the flow of the brake fluid in the passage 357. Thus, when the brakes are applied pressurised fluid will be supplied to the brake fluid inlet 345. Pressurised fluid will then flow along the first brake fluid passage 356 in the direction of arrow A to the piston and cylinder assemblies 354 in the limb 352.

Pressurised fluid will also flow through the non-return valve 358 and along the passage 357 in the direction of arrow B to the piston and cylinder assemblies 355 in the other limb 353.

When the brakes are released, brake fluid will be drawn from the brake fluid outlet 342. The brake fluid within the second brake fluid passage 357 is constrained to move in the direction of arrow B because of the nonreturn valve 358 and hence wi 11 tend to move towards and out ofthe outlet 342. The brake fluid in the passage 356 is prevented from returning through the inlet 345 because of the non-return val ve whi ch acts between the first chamber of the master cylinder 21 l and the brake application line 236 and will tend to flow in the direction of arrow towards the outlet 342.

The non-return valve 358 is shown in more detail in Figure 9 and has a ball 359 which is biased into engagement with a valve seat 360 by a spring 361. 'I'he bias force of the spring lo 361 is selected so as to be sufficient to hold the ball in contact with the valve seat when the brakes are released to inhibit movement of the fluid from the passage 356 into the passage 357.

The calipers shown in Figures 7 to 9 are only examples which may modified. In 2.

particular, additional non-return valve means could be used to further control movement I 5 of the bralce fluid within the calipers. For example, non-return valves could be provided in both fluid passages in the calipers.

It should be understood that the use of non-return valves to control the circulation of brake fluid within a bralce having two or more actuators connected in parallel is not limited to application to disc brakes having calipers of the opposed piston type. This aspect of the invention could equally be applied to disc braces having any form of caliper in which two or more actuators are connected in parallel. Indeed this aspect of the invention can also be applied to drum brakes having two or more brake cylinders connected in parallel.

It should also be understood that brakes in accordance with the second aspect of the 2s invention, such as the those described above in relation to Figures 7 to 9, are not limited to use with brake circuits as described above in relation to Figures 1 to 6 but can equally be used in any brake circuit which circulates brake fluid from the master cylinder to the brake means via an application line and back to the master cylinder via a return line. For example, they can be used brake circuits in accordance with US patents 5310252 and 5350223. . .

Claims

1. A brake circuit comprising brake means connected with a master cylinder via a brake application line and a brake return line, the brake circuit comprising non return valve means arranged such that when the brake master cylinder is actuated to apply the brake means, a quantity of brake fluid Is delivered to the brake means via the braLc application line and that when the brake master cylinder is released, a quantity of brake fluid is drawn from the brake means via the brake return line, the brake means comprising at least one brake having two or more brake actuators connected in parallel, characterised in that further non-return valve means are 0 provided to prevent or reduce circulation of the brake fluid between the parallel connected actuators of the brake.

2. A brake circuit as claimed in claim 1, in which the at least one brake is a disc brake having a brake caliper, the caliper comprising a body having at least two piston and cylinder assemblies, the caliper further comprising a fluid inlet and a fluid outlet, the inlet and outlet being connected in parallel by two fluid passages, one passage supplying fluid to at least one of the piston and cylinder assemblies, ., the other passage supplying fluid to at least one other piston and cylinder assembly, in which a non-return valve is provided in one of the passageways.

3. A brake circuit according to claim 2, in which a non-return valve is provided in each fluid passage.

4. A brake circuit according to claim 2 or claim 3, in which the caliper body comprises two limbs, each limb having at least one piston and cylinder assembly, one of the fluid passages supplying Buid to the at least one piston and cylinder assembly in one of the limbs, the other passage supplying fluid to the at least one piston and cylinder assembly in the other limb.

5. A brake circuit according to claim 4, in which the inlet and outlet are provided in one of the limbs, and the fluid passageway which supplies fluid to the or each piston assembly of the other limb comprises a pipe which extends externally of the body of the caliper.

6. A brake circuit according to claim 5, in which a non-return valve is provided between the inlet and a union to which one end of the external pipe is connected.

7. A brake circuit according to claim 1, in which the at least one brake is a drum brake comprising two or more brake cylinders connected in parallel.

8. A brake circuit constructed and arranged substantially as hereinbefore described with reference to and as shown in Figures 1 to 4 or Figures 5 & 6, when modified 0 by Figure 7 or Figures 8 & 9 of the accompanying drawings.

9. A brake for use in a brake circuit in accordance with any one of claims I to 8, the brake comprising, two or more actuators connected in parallel and non-retrurn valve means arranged to prevent or at least reduce circulation of brake fluid within the brake.

10. A brake according to claim 9, in which the brake comprises a disc brake caliper having at least two piston and cylinder assemblies, the caliper further comprising a fluid inlet and a fluid outlet, the inlet and outlet being connected in parallel by two fluid passages, one passage supplying fluid to at least one of the piston and cylinder assemblies, the other passage supplying fluid to at least one other piston and cylinder assembly, in which a non-return valve is provided in one of the passageways.

1 1. A brake according to claim 10, in which a non-return valve is provided in each fluid passage.

12. A brake according, to claim 10 or claim 1 1, in which the caliper includes a body having two limbs, each limb having at least one piston and cylinder assembly, one of the fluid passages supplying fluid to the at least one piston and cylinder assembly in one of the limbs, the other passage supplying fluid to the at least one piston and cylinder assembly in the other limb.

13. A brake according to claim 12, in which the inlet and outlet are provided in one of the limbs, and the fluid passageway which supplies fluid to the or each piston assembly of the other limb comprises a pipe which extends externally of the body ol the caliper.

14. A brake according to claim 13, in which a non-return valve is provided between l o the inlet and a union to which one end of the external pipe is connected.

15. A brake according to claim 10, comprising a drum brake having two or more brake cylinders connected in parallel.

16. A brake constructed and arranged substantially as hereinbefore described with reference to and as shown in Figure 7 or Figures 8 & 9 of the accompanying drawings.