GB2253884A

GB2253884A - Dual master cylinder

Info

Publication number: GB2253884A
Application number: GB9105965A
Authority: GB
Inventors: Henri Savidan
Original assignee: General Motors France SA
Current assignee: General Motors France SA
Priority date: 1991-03-21
Filing date: 1991-03-21
Publication date: 1992-09-23
Also published as: GB9105965D0

Abstract

A dual master cylinder (10) for the braking system of a motor vehicle is formed in the surface of the bore adjacent an elastomeric cup seal (52) on at least the secondary piston (44) with a channel (55) providing a fluid passage between the high pressure chamber and the low pressure chamber of the secondary portion during a rest mode but being isolated from the high pressure chamber during an apply mode. A similar channel may be provided for the primary piston. The channel prevents damage to the cup seal (52) when back pressure is generated by ABS control. <IMAGE>

Description

DUAL MASTER CYLINDER This invention relates to a dual master cylinder for the hydraulic braking system of a motor vehicle.

Dual master cylinders are well known, and comprise a primary portion and a secondary portion each comprising a low pressure chamber and a high pressure chamber. Each portion also comprises a piston, with the pistons being aligned. The primary piston and the secondary piston are slidably secured together such as to have a maximum relative separation. A primary spring is compressed between the primary piston and the secondary piston. A ring stop, engageable by the primary piston, retains the various components in the dual master cylinder. A secondary spring acts on the secondary piston to bias the pistons towards the ring stop. Each portion is supplied with hydraulic fluid to its low pressure chamber from a reservoir by way of a compensation port.Elastomeric cup seals mounted on the pistons allow passage of hydraulic fluid from the low pressure chambers to the high pressure chambers (but not flow in the reverse direction) to compensate for return movement of the piston and for brake pad or shoe wear. A dilation port connects each high pressure chamber to its respective reservoir to allow excess fluid (generated by thermal expansion, etc.) to flow back to its respective reservoir. The dilation ports are, necessarily, small to reduce the deadstroke of the dual master cylinder (that is, loss of stroke between brake pedal movement and pressure build up), and to reduce the risk of damaging the elastomeric cup seals as they pass over the dilation port opening during movement of the pistons.This arrangement is such that in usual circumstances, on brake pedal depression, the primary piston passes its associated dilation port to seal it from its associated high pressure chamber; the secondary piston then passes its associated dilation port; the fluid pressure in the high pressure chamber of the secondary portion then begins to increase; and then the fluid pressure in the high pressure chamber of the primary portion begins to rise. The use of a dual master cylinder in a motor vehicle provides two independent hydraulic circuits (a primary circuit and a secondary circuit integral with the primary portion and the secondary portion respectively) for the braking system. This ensures that the brakes can still be applied even in the event that one of the circuits should fail, such as due to a leakage of hydraulic fluid.

Whilst this known arrangement works satisfactorily on motor vehicles having a standard braking system, problems can arise on motor vehicles fitted with ABS (anti-lock braking systems), and in particular to back-pressure ABS in which hydraulic fluid can be pumped back to the high pressure chambers during operation of ABS. This action can result in very high fluid pressures being generated within the high pressure chambers. If, when ABS comes into operation, an elastomeric cup seal is positioned over a dilation port opening, the high pressure in the high pressure chamber can force the cup seal into the dilation port and damage it. Any such damage can result in a failure in at least one of the circuits. In usual arrangements, the Primary piston passes its corresponding dilation port before the secondary piston passes its corresponding dilation port on application of the vehicle brakes.

During ABS operation, therefore, it is more likely that the elastomeric cup seal on the secondary piston would be damaged, rather than the cup seal on the primary piston. It is, however, also possible in certain circumstances for the elastomeric cup seal on the primary piston to be so damaged. Suitable alternative arrangements have been proposed, but these have tended to involve extending the length of the master cylinder or the use of complicated valving arrangements.

It is an object of the present invention to overcome the above mentioned problems.

To this end, a dual master cylinder in accordance with the present invention comprises a bore having an open end and a closed end; a primary portion including a primary piston slidable in the bore, a low pressure chamber within the bore and defined by the shape of the primary piston, and a compensation port opening into the low pressure chamber and connectable with a primary fluid reservoir; and a secondary portion including a secondary piston slidable in the bore, a low pressure chamber within the bore and defined by the shape of the secondary piston, and a compensation port opening into the low pressure chamber and connectable with a secondary fluid reservoir; the primary portion including a high pressure chamber within the bore between the primary piston and the secondary piston, and the secondary portion including a high pressure chamber within the bore between the secondary piston and the closed end of the bore; a first seal being mounted on the primary piston between the low and high pressure chambers of the primary portion; a second seal being mounted on the secondary piston between the low and high pressure chambers of the secondary portion; the high pressure chamber of the primary portion being fluidly connectable with the primary fluid reservoir by primary dilation means; the high pressure chamber of the secondary portion being fluidly connectable with the secondary fluid reservoir by secondary dilation means; the secondary dilation means being defined by a secondary channel formed in the surface of the bore adjacent the second seal, the secondary channel having a predetermined overall length such as to extend across a portion of the outer edge of the second seal between one side thereof and the other side thereof, thereby providing a fluid passage between the high pressure chamber and the low pressure chamber of the secondary portion during a rest mode of the dual master cylinder, but be isolated from the high pressure chamber of the secondary portion by the second seal during an apply mode thereby preventing fluid flow through the secondary channel during the apply mode.

In the present invention, the secondary channel performs the function of the previously known dilation port for the secondary portion, and also provides a means for compensating for any reduction of hydraulic fluid in the high pressure chamber in the secondary portion. By removing the previously known dilation port from the secondary portion, the risk of potential damage of the second seal is substantially removed.

The second seal is preferably an elastomeric cup seal.

Preferably, the secondary channel has a predetermined cross-section such that a gap of minimal cross-sectional area exists between the outer surface of the second seal and the surface of the secondary channel in the rest mode to keep the deadstroke of the dual master cylinder as low as possible.

The secondary channel preferably has a leading edge positioned between the second seal and the closed end of the bore during the rest mode, the secondary channel having a minimal length between its leading edge and the second seal to keep the deadstroke of the dual master cylinder as low as possible. Preferably, the leading edge of the secondary channel is tapered. The secondary channel preferably extends from its leading edge to the compensation port in the secondary portion.

Preferably, the secondary channel is formed by stamping. Alternatively, the secondary channel may be formed by machine scratching.

The primary dilation means preferably comprises a primary channel formed in the surface of the bore adjacent the first seal, the primary channel having a predetermined overall length such as to extend across a portion of the outer edge of the first seal between one side thereof and the other side thereof, thereby providing a fluid passage between the high pressure chamber and the low pressure chamber of the primary portion during the rest mode of the dual master cylinder, but be isolated from the high pressure chamber of the primary portion by the first seal during the apply mode thereby preventing fluid flow through the primary channel during the apply mode.

In this arrangement, the primary channel performs the function of the previously known dilation port for the primary portion, and also provides a means for compensating for any reduction of hydraulic fluid in the high pressure chamber in the primary portion. By removing the previously known dilation port from the primary portion, the risk of potential damage of the first seal is substantially removed.

The first seal is preferably an elastomeric cup seal.

Preferably, the primary channel has a predetermined cross-section such that a gap of minimal cross-sectional area exists between the outer edge of the first seal and the surface of the primary channel in the rest mode to keep the deadstroke of the dual master cylinder as low as possible.

The primary channel preferably has a leading edge positioned between the first seal and the closed end of the bore during the rest mode, the primary channel having a minimal length between its leading edge and the first seal to keep the deadstroke of the dual master cylinder as low as possible. Preferably, the leading edge of the primary channel is tapered. The primary channel preferably extends from its leading edge to the compensation port in the primary portion.

Preferably, the primary channel is formed by stamping. Alternatively, the primary channel may be formed by machine scratching.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a dual master cylinder in accordance with the present invention with the various components in the rest mode; Figure 2 is an enlarged cross-sectional view of the dual master cylinder of Figure 1 in the area of the second; Figure 3 is a cross-sectional view on the line III-III of Figure 2; Figure 4 is an enlarged cross-sectional view of the dual master cylinder of Figure 1 in the area of the first seal; Figure 5 is a similar view to that of Figure 2 with the dual master cylinder in the apply mode; Figure 6 is a similar view to that of Figure 4 with the dual master cylinder in the apply mode; and Figures 7 and 8 are alternative arrangements of Figure 4 for use in a dual master cylinder in accordance with the present invention.

Referring to Figure 1, the dual master cylinder 10 comprises a primary portion 12 and a secondary portion 14. The primary portion 12 is connected to, and is part of, a primary circuit of the braking system of a motor vehicle. Similarly, the secondary portion 14 is connected to, and is part of, the secondary circuit of the braking system.

The primary portion 12 comprises a primary piston 16 axially slidable within a bore 18 having a closed end 17 and an open end 19 in the dual master cylinder 10, and movable by a pushrod 15 actuated by the vehicle operator by pressing on the brake pedal (not shown) of the braking system. The pushrod 15 passes through the open end 19 of the bore 18 to act on the primary piston 16. The primary piston 16 has a reduced diameter portion 20 between its ends to define a low pressure chamber 22 within the bore 18 for the primary portion 12. The primary portion 12 also includes a high pressure chamber 30 within the bore 18. The low pressure chamber 22 is connected to a primary fluid reservoir (not shown) by way of a compensation port 24.An elastomeric cup seal (first seal) 26 which moves with the primary piston 16 allows hydraulic fluid to flow from the low pressure chamber 22 to the high pressure chamber 30 to compensate for pressure differentials between the low and high pressure chambers 22,30 respectively, on return movement of the primary piston 16 (after application of the brakes), and for brake pad or shoe wear. The elastomeric cup seal 26, however, prevents flow of hydraulic fluid from the high pressure chamber 30 back to the low pressure chamber 22 during the apply mode of the dual master cylinder 10. A ring stop 35 mounted in the bore 18 adjacent the open end 19 retains the primary piston 16 within the bore. An elastomeric cup seal 28 positioned between the ring stop 35 and the open end 19 provides a fluid tight seal between the pushrod 15 and the bore 18 of the dual master cylinder 10.A spring retainer cage 36 is mounted within the high pressure chamber 30. A number of resilient fingers 41 extend away from one end 37 of the spring retainer cage 36, each of which has a lip 38 engageable with a shoulder 40 on an extended portion 42 of a secondary piston 44 (described in more detail below). The lips 38 on the resilient fingers 41 make a snap fit over the shoulder 40 on the extended portion 42 to secure the spring retainer cage 36 to the secondary piston 44, but to allow the spring retainer cage to slide along the extended portion 42. A primary spring 34 is precompressed and positioned between the secondary piston 44 and the spring retainer cage 36. The primary spring 34 biases the other end 43 of the spring retainer cage 36 into engagement with the primary piston 16.This arrangement is such that, in the rest mode, the primary spring 34 holds the primary and secondary pistons 16,44 respectively at a predetermined maximum separation. An outlet port 39 connects the high pressure chamber 30 with the other components (not shown) of the primary circuit of the braking system.

The secondary portion 14 comprises the secondary piston 44, the extended portion 42 of which extends into the high pressure chamber 30 of the primary portion 12. The secondary piston 44 is also slidably mounted in the bore 18 (such that it is axially aligned with the primary piston 16), and has a reduced diameter portion 46 between its ends defining a low pressure chamber 48 within the bore 18 for the secondary portion 14. A compensation port 50 connects the low pressure chamber 48 with a secondary fluid reservoir (not shown). The secondary portion 14 also includes a high pressure chamber 56 within the bore 18. An elastomeric cup seal (second seal) 52 is mounted on the secondary piston 44 to move therewith.

Elastomeric cup seal 52 allows hydraulic fluid to flow from the low pressure chamber 48 to the high pressure chamber 56 to compensate for pressure differentials between the low and high pressure chambers 48,56 respectively, on return movement of the secondary piston 44 (after application of the brakes), and for brake pad or shoe wear. The elastomeric cup seal 52, however, prevents flow of hydraulic fluid from the high pressure chamber 56 back to the low pressure chamber 48 during the apply mode of the dual master cylinder 10. Another elastomeric cup seal 54 mounted on the secondary piston 44 allows hydraulic fluid to flow from the low pressure chamber 48 to the high pressure chamber 30 of the primary portion 12, but not in the reverse direction. A secondary spring 58 is positioned within the high pressure chamber 56 and acts on the secondary piston 44 to bias an assembly of the secondary piston, primary spring 34, spring retainer cage 36, and primary piston 16 towards the open end 19 of the bore 18. The primary piston 16 engages the ring stop 35 in the rest mode to retain the assembly in the bore 18. The primary spring 34 is stronger than (usually of the order of twice as strong) the secondary spring 58 to ensure the whole assembly moves together on initial application of the vehicle brakes, as described below. An outlet port 60 in the high pressure chamber 56 provides a fluid connection with the other components of the secondary circuit.

The dual master cylinder 10 as thus far described is known. When the brake pedal (not shown) is depressed to apply the vehicle brakes, the pushrod 15 moves in the direction A and acts on the primary piston 16 to move the primary piston, the spring retainer cage 36, and, due to the primary spring 34 being stronger than the secondary spring 58, the secondary piston 44 relative to the bore 18 away from the open end 19 against the action of the secondary spring. Such movement of the secondary piston 44 pressurises the hydraulic fluid in the high pressure chamber 56 to apply the vehicle brakes by way of the secondary circuit. Further, such movement of the primary piston 16 pressurises the hydraulic fluid in the high pressure chamber 30 to apply the vehicle brakes by way of the primary circuit. Release of the brake pedal causes the above movement to be reversed.

However, the biasing effect of the secondary spring 58 is such that the secondary and primary pistons 44,16 respectively may move back quicker than the returning hydraulic fluid. To compensate for the 'shortfall' in hydraulic fluid in the high pressure chambers 30,56, hydraulic fluid flows past the elastomeric cup seals 26,52 respectively from the low pressure chambers 22,48 respectively. Similarly, any shortfall of hydraulic fluid in the high pressure chambers 30,56 due to wear of the brake pads or brake shoes can be compensated for in this way.

In accordance with the present invention, the secondary portion 14 also includes secondary dilation means in the form of a secondary channel 55 formed in the surface of the bore 18 adjacent the elastomeric cup seal 52. The high pressure chamber 56 is connected to the secondary fluid reservoir by way of the secondary channel 55 in the rest mode. The secondary channel 55 acts as a fluid passage and allows excess hydraulic fluid (generated by thermal expansion, etc.) to flow back to the secondary fluid reservoir to ensure there is no residual fluid pressure in the high pressure chamber 56. Any build up in fluid pressure in the high pressure chamber 56 (due to thermal expansion etc.) when the dual master cylinder 10 is in the rest mode is dilated to the secondary fluid reservoir by way of the secondary channel 55.

Further, the primary portion 12 also includes primary dilation means in the form of a primary channel 32 formed in the surface of the bore 18 adjacent the elastomeric cup seal 26. The high pressure chamber 30 is connected to the primary fluid reservoir by way of the primary channel 32 in the rest mode. The primary channel 32 acts as a fluid passage and allows excess hydraulic fluid (generated by thermal expansion, etc.) to flow back to the primary fluid reservoir to ensure there is no residual fluid pressure in the high pressure chamber 30. Any build up in fluid pressure in the high pressure chamber 30 (due to thermal expansion etc.) when the dual master cylinder 10 is in the rest mode is dilated to the primary fluid reservoir by way of the primary channel 32.

The primary and secondary channels 32,55 can be formed by stamping or by machine scratching.

Each channel 32,55 extends substantially longitudinally across a portion of the outer edge 27,53 of the respective elastomeric cup seals 26,52 from one side 65,67 thereof to the other side 66,68 thereof respectively, and has a predetermined overall length. This arrangement allows fluid to flow between the high pressure chambers 30,56 and their respective low pressure chambers 22,48 during the rest mode (Figures 2 and 4), but be sealed from the high pressure chambers during the apply mode (Figures 5 and 6). In the present embodiment, each channel 32,55 extends from a leading edge 62,64 respectively towards the open end 19 of the bore 18 and opens into the respective compensation port 24,50. The leading edge 62,64 of the primary and secondary channel 32,55, respectively, is tapered to avoid damage to the respective elastomeric cup seal 26,52 as it moves along the bore 18.

In the rest position shown in Figures 2 and 4, a gap exists between the surface of the primary and secondary channels 32, 55 and the outer edge 27,53 of the respective elastomeric cup seals 26,52.

Hydraulic fluid can therefore flow between the primary fluid reservoir and the high pressure chamber 30 by way the low pressure chamber 22 and the primary channel 32. Similarly, hydraulic fluid can flow between the secondary fluid reservoir and the high pressure chamber 56 by way the low pressure chamber 48 and the secondary channel 55.

Each channel 32,55 also has a minimal length L,L' (Figures 2 and 4) between its leading edge 62,64 and the respective elastomeric cup seal 26,52 in the rest mode, and has a cross-section (Figure 3) such that the cross-sectional area A of the gap between the outer edge 27,53 of the elastomeric cup seal 26,52 and the surface of the corresposnding channel 32,55 is kept to a minimum.

Further, the outer edges 27,53 of the elastomeric cup seals 26,52 engage and seal with the leading edges 62,64 of the corresponding channel 32,55 before sealing with the bore 18 (on application of the vehicle brakes). This arrangement keeps the deadstroke of the dual master cylinder 10 (that is, loss of stroke between brake pedal movement and pressure build up) as small as possible.

When the brake pedal (not shown) is depressed (to apply the vehicle brakes), pushrod 15 moves in the direction X (Figure 1) to move the primary piston 16 and the secondary piston 44 in the same direction, that is, away from the open end 19, to compress the secondary spring 58 (due to the primary spring 34 acting on the secondary piston).

This movement of the primary and secondary pistons 16,44 also moves the elastomeric cup seals 26,52 towards the closed end 17 of the bore 18 such that their outer edges 27,53 totally engage the surface of the bore (Figures 5 and 6). In this apply mode, the primary and secondary channels 32,55 are isolated from their respective high pressure chambers 30,56.

This movement of the secondary piston 44 pressurises the hydraulic fluid in the high pressure chamber 56 to apply the vehicle brakes by way of the secondary circuit. Continued movement of the primary piston 16 in the direction X begins to compress the primary spring 34 and pressurises the hydraulic fluid in the high pressure chamber 30 to apply the vehicle brakes by way of the primary circuit.

When the braking effort is released, the pressure of the hydraulic fluid and the bias of the primary and secondary springs 34,58 act on the primary and secondary pistons 16,44 to move them back in the opposite direction to direction X to the rest position shown in Figures 2 and 4.

The primary and secondary channels 32,55 allow passage of hydraulic fluid from the primary and secondary fluid reservoirs, respectively into the high pressure chambers 30,56 to compensate for a shortfall of hydraulic fluid in the high pressure chambers due to wear of the brake pads or brake shoes. This compensating effect enhances the same effects provided by the elastomeric cup seals 26,52.

Further still, the primary and secondary channels 32,55 allow reverse flow (dilation) of hydraulic fluid should there be an unintentional build up of fluid pressure in the high pressure chambers 30,56 due to thermal expansion, etc. The primary and secondary channels 32,55 therefore fulfil the same purpose as the previously known dilation ports, and no such dilation ports are required. Where the braking system includes ABS, when ABS operates a flow of hydraulic fluid is sent back to the high pressure chambers 30,56 increasing the fluid pressure therein.

As no dilation ports are present, no damage can occur to the elastomeric cup seals 26,52.

As well as overcoming the problems associated with prior known dual master cylinders, the present invention has the additional advantage that all of the components within the bore 18 of the dual master cylinder 10 can be assembled as a complete sub-assembly prior to insertion in the bore, and can be inserted in any orientation as there is no requirement to align it with a component inserted through the housing of the dual master cylinder (which also means there is no possibility of fluid leakage around such a component). Still further, the wall of the dual master cylinder does not have to be pierced to provide a dilation port, again easing manufacture. Further still, the channel arrangement can be incorporated into the dual master cylinder without any increase in its length, and the arrangement is very simple.A significant advantage of the present invention is that the dual master cylinder has ABS compatibility for very low cost.

Alternative arrangements, which can be used for either or both of the primary and secondary channels are shown in Figures 7 and 8. In the Figure 7 arrangement, the channel 70 extends between the low pressure chamber 72 and the high pressure chamber 74 in the rest mode shown, but does not open into the compensation port 76. In the Figure 8 arrangement, the channel 80 extends for a predetermined overall length from one side to the other side of the elastomeric cup seal 82 while in the rest mode shown.

Fluid can flow between the low pressure chamber 84 and the high pressure chamber 86 by passing through the channel 80 and seeping past the piston 88.

Although the channels 32,55 have been shown as extending substantially longitudinally, they may extend at an angle to the longitudinal direction of the bore 18. Whilst the present invention has been described using a primary channel as the dilation means for the primary portion, other forms of.primary dilation means may be used, such as the check valve arrangement described in our GB patent application no. 8920874.8.

Claims

Claims:

1. A dual master cylinder for the braking system of a motor vehicle comprising a bore having an open end and a closed end; a primary portion including a primary piston slidable in the bore, a low pressure chamber within the bore and defined by the shape of the primary piston, and a compensation port opening into the low pressure chamber and connectable with a primary fluid reservoir; and a secondary portion including a secondary piston slidable in the bore, a low pressure chamber within the bore and defined by the shape of the secondary piston, and a compensation port opening into the low pressure chamber and connectable with a secondary fluid reservoir; the primary portion including a high pressure chamber within the bore between the primary piston and the secondary piston, and the secondary portion including a high pressure chamber within the bore between the secondary piston and the closed end of the bore; a first seal being mounted on the primary piston between the low and high pressure chambers of the primary portion; a second seal being mounted on the secondary piston between the low and high pressure chambers of the secondary portion; the high pressure chamber of the primary portion being fluidly connectable with the primary fluid reservoir by primary dilation means; the high pressure chamber of the secondary portion being fluidly connectable with the secondary fluid reservoir by secondary dilation means; the secondary dilation means being defined by a secondary channel formed in the surface of the bore adjacent the second seal, the secondary channel having a predetermined overall length such as to extend across a portion of the outer edge of the second seal between one side thereof and the other side thereof, thereby providing a fluid passage between the high pressure chamber and the low pressure chamber of the secondary portion during a rest mode of the dual master cylinder, but be isolated from the high pressure chamber of the secondary portion by the second seal during an apply mode thereby preventing fluid flow through the secondary channel during the apply mode.

2. A dual master cylinder as claimed in Claim 1, wherein the second seal is an elastomeric cup seal.

3. A dual master cylinder as claimed in Claim 1 or Claim 2, wherein the secondary channel has a predetermined cross-section such that a gap of minimal cross-sectional area exists between the outer edge of the second seal and the surface of the secondary channel in the rest mode.

4. A dual master cylinder as claimed in any one of Claims 1 to 3, wherein the secondary channel has a leading edge positioned between the second seal and the closed end of the bore during the rest mode, the secondary channel having a minimal length between its leading edge and the second seal.

5. A dual master cylinder as claimed in Claim 4, wherein the leading edge of the secondary channel is tapered.

6. A dual master cylinder as claimed in Claim 4 or Claim 5, wherein the secondary channel extends from its leading edge to the compensation port in the secondary portion.

7. A dual master cylinder as claimed in any one of Claims 1 to 6, wherein the secondary channel is formed by stamping.

8. A dual master cylinder as claimed in any one of Claims 1 to 6, wherein the secondary channel is formed by machine scratching.

9. A dual master cylinder as claimed in any one of Claims 1 to 8, wherein the primary dilation means comprises a secondary channel formed in the surface of the bore adjacent the first seal, the primary channel having a predetermined overall length such as to extend across a portion of the outer edge of the first seal between one side thereof and the other side thereof, thereby providing a fluid passage between the high pressure chamber and the low pressure chamber of the primary portion during the rest mode of the dual master cylinder, but be isolated from the high pressure chamber of the primary portion by the first seal during the apply mode thereby preventing fluid flow through the primary channel during the apply mode.

10. A dual master cylinder as claimed in any one of Claims 1 to 9, wherein the first seal is an elastomeric cup seal.

11. A dual master cylinder as claimed in Claim 9 or Claim 10, wherein the primary channel has a predetermined cross-section such that a gap of minimal cross-sectional area exists between the outer edge of the first seal and the surface of the primary channel in the rest mode.

12. A dual master cylinder as claimed in any one of Claims 9 to 11, wherein the primary channel has a leading edge positioned between the first seal and the closed end of the bore during the rest mode, the primary channel having a minimal length between its leading edge and the first seal.

13. A dual master cylinder as claimed in Claim 12, wherein the leading edge of the primary channel is tapered.

14. A dual master cylinder as claimed in Claim 12 or Claim 13, wherein the primary channel extends from its leading edge to the compensation port in the primary portion.

15. A dual master cylinder as claimed in any one of Claims 9 to 14, wherein the primary channel is formed by stamping.

16. A dual master cylinder as claimed in any one of Claims 9 to 14, wherein the primary channel is formed by machine scratching.

17. A dual master cylinder substantially as hereinbefore described with reference to, and as shown in, Figures 1 to 6 or Figure 7 or Figure 8 of the accompanying drawings.