Dual master cyclinder with compensation valve

A dual master cylinder (10) for the braking system of a motor vehicle comprising a bore (18), a primary portion (12) and a secondary portion, in which the primary piston (16) comprises an outer piston (65) slidable in the bore (18), and a central piston (78) slidable in a piston bore (64) defined by the outer piston, the piston bore having a shoulder (72) engageable with an abutment face (98) on the central piston to define a check valve (62), the piston bore providing a fluid passage (70,96,68) between the high pressure chamber (30) and the low pressure chamber (22) of the primary portion, the check valve allowing hydraulic fluid to flow through the fluid passage during a rest mode or a release mode of the dual master cylinder, but preventing such flow during an apply mode. Prevents damage to elastomeric cup seal (26) when back pressure is generated by ABS due to the absence of a dilation port in the primary portion, which is no longer required due to the presence of the check valve.

This invention relates to a dual master cylinder for the hydraulic braking 
system of a motor vehicle. Other dual master cylinders are shown in U.S. 
Ser. No. 391,930 and G-6623 Dual Master Cylinder and G-6635 Dual Master 
Cylinder with Compensation filed on even date herewith. 
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. 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 could be 
damaged, rather than the cup seal on the primary piston. However, it is 
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. 
It is an object of the present invention to overcome the above mentioned 
problem. 
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 seal being mounted on the primary piston between the low and 
high pressure chambers of the primary portion; a seal being mounted on the 
secondary piston between the low and high pressure chambers of the 
secondary portion; the high pressure chamber of the secondary portion 
being fluidly connectable with the primary fluid reservoir by dilation 
means; the primary piston comprising an outer piston slidable in the bore, 
and a central piston slidable in a piston bore defined by the outer 
piston, the piston bore having a shoulder engageable with an abutment face 
on the central piston to define a check valve, the piston bore providing a 
fluid passage between the high pressure chamber and the low pressure 
chamber of the primary portion, the check valve allowing hydraulic fluid 
to flow through the fluid passage during a rest mode or a release mode of 
the dual master cylinder, but preventing such flow during an apply mode. 
In the present invention, the check valve 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, potential damage of the seal on 
the primary piston is removed. 
The seal on the primary piston is preferably an elastomeric cup seal. In 
this case, both the elastomeric cup seal and the check valve can provide 
the compensating effect. The seal on the secondary piston is preferably an 
elastomeric cup seal. 
Preferably, the check valve comprises an elastomeric ring seal positioned 
between the abutment face and the shoulder. The elastomeric ring seal is 
preferably a metal washer having an elastomeric ring bonded to its inner 
circumference. Alternatively, the elastomeric ring seal may comprise an 
elastomeric ring positioned in, and protruding from, a groove in the 
abutment face or the shoulder. As a further alternative, the elastomeric 
ring seal may comprise an elastomeric ring bonded to the abutment face or 
the shoulder. As a still further alternative, the elastomeric ring seal 
may comprise an elastomeric ring positioned in, and protruding from, a 
circumferentially extending groove in the central piston adjacent the 
abutment face. In this latter case, the shoulder preferably includes 
abutment means engageable by the abutment face to prevent over-compression 
of the elastomeric ring. 
Preferably, the piston bore comprises a small diameter portion which opens 
into the high pressure chamber of the primary portion and a large diameter 
portion, the shoulder being positioned between the large diameter portion 
and the small diameter portion; an aperture extends through the outer 
piston between the large diameter portion and the low pressure chamber of 
the primary portion; and the central piston comprises a main body slidably 
positioned within the large diameter portion of the piston bore, and a 
secondary body slidably positioned in the small diameter portion of the 
piston bore, the abutment face being positioned between the main body and 
the secondary body, the secondary body having a channel in its surface; 
the fluid passage being defined by the aperture, the large diameter 
portion, and the channel. In this case, the axial length of the secondary 
body is preferably greater than the axial length of the small diameter 
portion by a predetermined amount to define the maximum separation of the 
abutment face and the shoulder. 
The central piston preferably has a shaped end to receive a pushrod. 
Preferably, where the seal on the secondary piston is an elastomeric cup 
seal, the dilation means in the secondary portion is a dilation port. 
Alternatively, the dilation means may be a check valve means.

Referring to FIGS. 1 to 4, 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 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. 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. Elastomeric cup seals 52,54 are 
mounted on the secondary piston 44 to move therewith. One of the 
elastomeric cup seals 52 allows hydraulic fluid to flow from the low 
pressure chamber 48 to the high pressure chamber 56, but not in the 
reverse direction, 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. Similarly, the other elastomeric cup seal 54 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. The high pressure chamber 56 is connected to the secondary 
fluid reservoir by way of a dilation port 55 in a rest mode of the 
secondary piston 44, that is, when the brake pedal is not depressed. The 
dilation port 55 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. 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 
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 isolates the dilation port 55 from the 
high pressure chamber 56, and pressurizes 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 pressurizes 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. 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 dilation 
port 55. 
In accordance with the present invention, the primary portion 12 also 
includes a check valve 62 (FIGS. 1 and 4). The check valve 62 is defined 
by the primary piston which is split into a central piston 78 and an outer 
piston 65. The central piston 78 is positioned within a piston bore 64 in 
the outer piston 65. The piston bore 64 comprises a small diameter portion 
66 which opens into the high pressure chamber 30, and a large diameter 
portion 68 which opens at one end into the small diameter portion 66, and 
which is connected to the low pressure chamber 22 by way of an aperture 70 
through the outer piston 65. A shoulder 72 connects the small diameter 
portion 66 with the large diameter portion 68 at said one end of the large 
diameter portion. The other end 69 of the large diameter portion 68 is 
open and directed towards the open end 19 of the bore 18. The central 
piston 78 comprises a main body 86 slidable within the large diameter 
portion 68 of the piston bore 64, and having a shaped portion 88 at one 
end which receives the pushrod 15, and a secondary body 92 extending from 
the other end of the main body, and making a sliding fit in the small 
diameter portion 66 of the piston bore. The axial length of the secondary 
body 92 is greater than the axial length of the small diameter portion 66 
of the piston bore 64. The secondary body 92 has an end face 93 directed 
towards and in engagement with the said other end 43 of the spring 
retainer cage 36. The secondary body 92 also has at least one channel 96 
extending along its surface. The outer piston 65 has one end 90 engageable 
with the ring stop 35, and the other end 91 engageable with said other end 
43 of the spring retainer cage 36. An abutment face 98 is defined on the 
central piston 78 where the secondary body 92 meets the main body 86 which 
is directed towards the shoulder 72 defined by the piston bore 64. An 
elastomeric ring seal 80 is positioned around the secondary body 92 
between the shoulder 72 and the abutment face 98. The elastomeric ring 
seal 80 (FIG. 7) comprises a steel washer 82 having an elastomeric ring 84 
bonded to its inner circumference. The check valve 62 thereby comprises 
the central piston 78, the elastomeric ring seal 80, and the outer piston 
65. 
In the rest position shown in FIGS. 1 and 4, the central and outer pistons 
78,65 respectively are biased towards the open end 19 of the bore 18 by 
the secondary spring 58. However, the outer piston 65 engages the ring 
stop 35 and the lips 38 on the resilient fingers 41 engage the shoulder 40 
on the extended portion 42 due to the bias of the primary spring 34, to 
leave the end face 93 of the central piston 78 substantially aligned with 
the said other end 91 of the outer piston 65. As a consequence, the 
secondary body 92 projects into the large diameter portion 68 of the 
piston bore 64, and a gap 89 having a predetermined size X exists between 
the abutment face 98 on the central piston 78 and the shoulder 72 defined 
by the piston bore 64. Hydraulic fluid can therefore flow between the 
primary fluid reservoir and the high pressure chamber 30 by way of 
compensation port 24, low pressure chamber 22, aperture 70, the large 
diameter portion 68 of the piston bore 64, and channel(s) 96. Aperture 70, 
channel(s) 96, and large diameter portion 68 thereby define a fluid 
passage between the low pressure chamber 22 and the high pressure chamber 
30. 
When the brake pedal (not shown) is depressed (to apply the vehicle 
brakes), pushrod 15 moves in the direction A (FIGS. 2 and 5) to move the 
central piston 78 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 44) due to the engagement of the end face 
93 of the secondary body 92 on the said other end 43 of the spring 
retainer cage 36. However, due to the resistance of the hydraulic fluid in 
the high pressure chamber 30 and the frictional effects of the elastomeric 
cup seal 26, the outer piston 65 does not move relative to the open end 19 
of the bore 18. When the central piston 78 has moved a distance X equal to 
the gap 89, the elastomeric ring seal 80 engages both the abutment face 98 
on the central piston 78 and the shoulder 72 in the piston bore 64. 
Further movement of the central piston 78 in the same direction A 
compresses the elastomeric ring 84 to form a fluid tight seal between the 
abutment face 98 and the shoulder 72, and hence seals the fluid passage 
70,96,68. Still further movement of the central piston 78 towards the 
closed end 17 pressurizes the hydraulic fluid in the high pressure chamber 
56 to apply the vehicle brakes by way of the secondary circuit, and 
pressurizes the hydraulic fluid in the high pressure chamber 30 to apply 
the vehicle brakes by way of the primary circuit During this action, the 
steel washer 82 of the elastomeric ring seal 80 engages the abutment face 
98 and the shoulder 72 to prevent over-compression of the elastomeric ring 
84 (to prevent damaging it), and to move the central piston 78 and the 
outer piston 65 together. Continued movement of the central piston 78 in 
the direction A begins to compress the primary spring 34 until the end 
face 93 of the central piston engages the extended portion 42 of the 
secondary piston 44. 
When the braking effort is released (FIGS. 3 and 6), the pressure of the 
hydraulic fluid and the bias of the secondary spring 58 act on the outer 
piston 65 and the central piston 78 to move them back in the direction B 
(opposite to direction A) to the rest position shown in FIGS. 1 and 4. 
However, as the primary spring 34 initially only exerts a biasing (return) 
force on the central piston 78, the central piston may move in the 
direction B quicker than the outer piston 65. As a consequence, the gap 89 
between the abutment face 98 and the shoulder 72 begins to reopen until 
the said other end 43 of the spring retainer cage 36 engages the said 
other end 91 of the outer piston 65, at which point the outer piston and 
the central piston 78 move back together, and the gap 89 returns to its 
predetermined size X. The outer piston 65 and the central piston 78 then 
continue to move back until the outer piston 65 engages the ring stop 35. 
The biasing effect of the secondary spring 58 is such that the primary 
piston 16 may move back quicker than the returning hydraulic fluid. The 
early reopening of the gap 89 allows hydraulic fluid to flow from the low 
pressure chamber 22 into the high pressure chamber 30 to compensate for 
this initial shortfall or lack of hydraulic fluid in the high pressure 
chamber Further, the gap 89 allows passage of hydraulic fluid from the 
primary fluid reservoir into the high pressure chamber 30 to compensate 
for a shortfall of hydraulic fluid in the high pressure chamber 30 due to 
wear of the brake pads or brake shoes. This compensating effect enhances 
the same effects provided by the elastomeric cup seal 26. Further still, 
the gap 89 allows reverse flow (dilation) of hydraulic fluid should there 
be an unintentional build up of fluid pressure in the high pressure 
chamber 30 due to thermal expansion, etc. The gap 89 therefore fulfills 
the same purpose as the dilation port 55 of the secondary portion 14, and 
no such port is required in the primary portion 12. Where the braking 
system includes ABS, when ABS operates a flow of hydraulic fluid is sent 
back to the high pressure chamber 30 increasing the fluid pressure 
therein. As no dilation port is present in the primary portion 12, no 
damage can occur to the elastomeric cup seal 26. 
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). Further still, the check valve arrangement can be incorporated 
into the dual master cylinder without any increase in its length, and the 
arrangement is very simple. 
An alternative embodiment of primary piston 16' and check valve 62' is 
shown in FIG. 8. In this case, the outer piston 65 and the central piston 
78' are substantially as described above. However, in this case, the 
elastomeric ring seal 100 comprises an elastomeric ring 102 which is 
positioned in, and protrudes from, a groove 104 in the shoulder 72' 
between the large diameter portion 68' and the small diameter portion 66' 
of the piston bore 64'. The elastomeric ring 102 may be bonded in position 
in the groove 104. Alternatively, the elastomeric ring may be positioned 
in, and protrude from, a groove in the abutment face 98' of the central 
piston. As a further alternative, the elastomeric ring may be simply 
bonded to the surface of the shoulder or the abutment face. 
A further alternative embodiment of primary piston 16" and check valve 62" 
is shown in FIG. 9. In this case, the outer piston 65" and the central 
piston 78" are substantially as described above. However, in this case, 
the elastomeric ring seal 110 comprises an elastomeric seal 112 which is 
positioned in, and protrudes from, a circumferentially extending groove 
114 in the secondary body 92" adjacent the abutment face 98". The 
elastomeric ring 112 may be bonded in position in the circumferentially 
extending groove 114. In this case, the shoulder 72" between the large 
diameter portion 68" and the small diameter portion 66" of the piston 64" 
preferably includes a step 116 to define abutment means During the apply 
mode of the dual master cylinder, the abutment face 98" engages the 
abutment means to prevent over-compression of the elastomeric ring 112. 
Whilst the present invention has been described in regard to a dual master 
cylinder having a check valve in the primary portion only, a check valve 
may also be positioned in the secondary portion 14 to replace the dilation 
port 55. This check valve may be a suitably modified version of the above 
described check valve. Preferably, however, this check valve is as 
described either in our other patent application no. Ref: MJD/467), or as 
described in our patent application no. (Ref: MJD/469), both filed the 
same day as the present application.