Brake master cylinder

In a master cylinder of the quick-fill type, communication between the quick-fill chamber (8) and the reservoir (13) is controlled by a valve assembly comprising a pressure responsive valve which opens when the pressure in the quick-fill chamber (8) reaches a predetermined level, and a one-way valve (20, 22) which allows free flow of fluid from the reservoir to the quick-fill chamber. The valve member (20) and seat (15) of the pressure responsive valve have confronting surfaces which define between them, when the valve is closed, a restrictive passage for pressure equalization purposes, and any dirt becoming trapped between the surfaces is washed away by the fluid flow on opening of the valve.

This invention is concerned with a master cylinder for an hydraulic braking 
system, and in particular with a master cylinder of the so-called "quick 
take-up" or "quick fill" type. 
A quick-fill master cylinder is one which on actuation initially supplies a 
large amount of fluid to the braking circuit so that clearances are 
rapidly taken up, and the brake shoes or pads are placed in contact with 
the associated brake drums or discs ready to apply the braking forces, 
with only minimal travel of the brake pedal itself. Generally, a quick 
fill master cylinder has a stepped bore and a piston with two head of 
different diameter sliding in respective parts of the bore. A large volume 
chamber is defined between the two piston heads and the bore while a lower 
volume chamber is defined within the bore in front of the smaller piston 
head and has an outlet leading to the brake circuit. When the brake pedal 
is depressed, the piston is urged forwards along the bore causing the 
volume of the large chamber to reduce, and braking fluid to be transferred 
from it to the lower volume chamber and hence to the braking circuit for 
taking up the clearances. Once the clearances have been taken up, the rise 
in pressure in the lower volume chamber prevents more fluid being 
transferred to it from the larger chamber, and instead on continued 
forward movement of the piston fluid is allowed to pass from the latter 
chamber to a reservoir of the master cylinder via a pressure responsive 
valve device. The piston movement pressurises the fluid in the lower 
volume chamber, which pressure is transmitted to the brake cylinders to 
apply the brakes. 
Quick fill master cylinders have the advantages of reducing the length of 
the master cylinder and brake pedal travel and of making a booster 
unnecessary in at least some applications. 
In any braking circuit the pedal travel required to achieve brake 
application should be the same irrespective of the rate at which the pedal 
is depressed. In addition, it is essential that the pressure chamber of 
the master cylinder should be allowed to "breathe", i.e. have some fluid 
communication with the reservoir when in its normal brakes-off condition, 
for example to allow for thermal expansion and contraction of the fluid in 
the braking circuit due to fluctuations in the ambient temperature when 
the vehicle is parked. Furthermore, in the case of quick fill master 
cylinders it is preferable that the pressure in the quick-fill, i.e. 
larger volume chamber be allowed to decay through a restricted passage 
communicating the chamber with the reservoir so that brake application is 
not resisted by this pressure causing the pedal to have a strange feel to 
it. 
The known quick-fill master cylinders fail to achieve all the above 
mentioned qualities and as a result all have their drawbacks. Typical of 
the prior art are the quick-fill master cylinders of U.S. Pat. Nos. 
4,086,770, 4,133,178 and 4,208,881. In the master cylinder disclosed in 
the first of these specifications, the quick-fill chamber communicates 
with the reservoir through a valve which is opened when the pressure in 
the low volume pressurising chamber reaches a particular level, and 
through a one-way valve which is arranged to permit return flow of fluid 
to the quick-fill chamber on the return stroke of the piston. A 
compensation port normally communicates the pressurising chamber with the 
reservoir to allow it to breathe. This master cylinder has two main 
disadvantages. Firstly, upon a very rapid actuation the pressure in the 
pressurising chamber can rise to a level at which the valve is opened 
before any pressure has been transmitted to the brakes themselves. As a 
result the fluid from the quick fill chamber passes to the reservoir and 
the quick fill operation is lost so that pedal travel is increased. 
Secondly, the compensation port is quickly placed in communication with 
the quick-fill chamber even at slow actuation rates, with the result that 
on slow actuation the fluid passes from the quick-fill chamber to the 
reservoir via the compensation port and once again the quick-fill 
operation is lost and pedal travel increased. 
In the master-cylinder of U.S. Pat. No. 4,133,178 the quick-fill chamber 
and compensation port communicate with the reservoir through a common 
passage which includes a valve and a restriction. In the normal brakes-off 
condition the piston acts on a stem of the valve to hold it open so that 
the pressurising chamber can breathe. When the piston moves forwards the 
valve closes under the action of a spring to allow sufficient pressure to 
be created in the quick-fill chamber to transfer fluid into the 
pressurising chamber. On a fast actuation the valve may not close due to 
the rapid pressure increase, but the restriction in the passage ensures 
that fluid is transferred to the pressurising chamber. The master cylinder 
still has its disadvantages however. There is no possibility for the 
pressure in the quick-fill chamber to decay so that resistance is felt at 
the pedal, especially upon slow actuation. In addition the piston 
displacement needed to close the valve leads to considerable lost travel 
of the pedal. In U.S. Pat. No. 4,208,881 an attempt has been made to avoid 
these drawbacks. The valve stem has been eliminated and a small bore 
passageway formed in the valve seat member to by-pass the valve. The 
restrictive passageway means that the pressurising chamber can still 
breathe, and also allows the pressure in the quick-fill chamber to decay 
under static pressure conditions. However, in order to be sufficiently 
restricting, the passageway must be so narrow that it is easily blocked by 
dirt particles. This drawback cannot be removed by making the passageway 
bigger because then, on slow actuation, the fluid from the quick-fill 
chamber would flow to the reservoir through the passageway and the quick 
fill operation would be lost. 
The present invention aims at reducing the drawbacks of the known 
quick-fill master cylinders and, accordingly, provides a master cylinder 
of the quick-fill type comprising a body having a stepped bore, a piston 
having opposite ends slidable in respective portions of the bore and 
defining therewith a low volume pressure chamber and a larger volume 
quick-fill chamber, a port in the body open to the quick-fill chamber and 
a compensation port in the body for communicating with the pressure 
chamber, a reservoir connected to said ports, and valve means controlling 
communication between the reservoir and the ports, the valve means 
including a pressure responsive valve arranged to open when the pressure 
in the quick-fill chamber reaches a predetermined level, and a one-way 
valve permitting free flow of fluid in the direction from the reservoir to 
the quick-fill chamber, the pressure responsive valve comprising a valve 
member and seat having co-operating confronting surfaces which move apart 
when the valve opens for fluid to flow therebetween in a direction 
substantially parallel to the said surfaces, and a restricting by-pass 
passage being defined between said confronting surfaces when the pressure 
responsive valve is closed. 
A master cylinder of this construction has the advantage that any dirt 
particles becoming trapped between the confronting surfaces which define 
between them the restricted passage, will be washed clear by fluid flowing 
under pressure to the reservoir on brake application at a sufficient rate 
to lift the valve member off the seat member. 
In one embodiment of the invention the pressure responsive valve is formed 
by a moulded plastics valve member having a substantially flat end surface 
adapted to cooperate with the bottom of a cup-shaped seat member received 
in and sealed to a boss on the master cylinder body. The valve member has 
a bore in which a ball is held trapped and cooperates with an annular seat 
provided on the valve member to define the one-way valve. The restricted 
passage is defined by three short feet on the flat end surface of the 
valve member which hold this surface at a small clearance of about 0.1 mm 
from the surface of the cup-shaped seat member. When the valve member 
lifts off its seat the flow of fluid washes the surfaces free of any 
trapped particles whereby the restricted passage is automatically 
self-cleaning. 
During normal driving most brake applications are at a slow rate, and if 
dirt should become trapped between the valve member and its seat so that 
they do not close together properly, a higher rate of application may be 
need to instigate the self-cleaning action for the restricted passage. As 
a result, for a few brake applications at least the pedal feel and travel 
could be different to that normally experienced by the driver, due to 
fluid passing at a higher rate than usual from the quick-fill chamber to 
the reservoir via the restricted passage. Of course, any variations in 
pedal characteristics during driving are preferably avoided. 
This possible drawback can be avoided if one of the two confronting 
surfaces between which the restrictive passage is defined, is provided by 
an element of elastomeric material. 
When the two components of the pressure responsive valve are urged together 
to define the restricted passage any dirt particles between their 
contacting surfaces will be pressed down into the elastomeric material, 
due to its resilient nature, and will not prevent the components coming 
together into correct sealing engagement to form the restricted passage. 
The self-cleaning, however, is not lost since the dirt particles can be 
washed clear when the components are subsequently opened apart again, e.g. 
upon brake application at a high rate. 
As the sealing engagement between the components is not impeded by any dirt 
becoming trapped between them, the pedal travel will always remain 
constant for all rates of pedal depression. 
Generally speaking it will not be satisfactory to provide the restricted 
passage by forming a groove in one of the confronting surfaces. In order 
to be sufficiently restrictive the groove would need to have a small 
cross-section which means, firstly, that it would easily become blocked by 
solid dirt particles and, secondly, such narrow grooves of accurately 
controlled size are costly to machine. 
However, with a master cylinder according to the present invention it has 
been found possible to employ a groove as the restricted passage without 
suffering the above disadvantages. Because the passage is defined between 
confronting surfaces a comparatively long groove of large cross-section 
can be provided in one of the surfaces and have the necessary flow 
resistance, the groove following a generally spiral path which preferably 
includes at least one substantially complete turn. 
If the master cylinder includes an element of elastomeric material as 
mentioned above, the spiral groove is conveniently formed in this element 
during moulding.

In FIG. 1 numeral 1 designates the master cylinder body in which there is 
provided a stepped bore with larger and smaller diameter portions 2, 3 
separated by a shoulder. A piston 4 received in the bore carries a seal 5 
at its forward end sliding in bore portion 2, and a second seal 6 at its 
rear end sliding in bore portion 3. A low volume pressurising chamber 7 is 
defined in bore portion 2 in front of the piston 4 and a larger volume, 
quick-fill chamber 8 is defined around the piston 4 between its seals 5, 
6. The body 1 has an integral boss 9, the interior of which communicates 
with the quick-fill chamber 8 through a port 10 and with the pressurising 
chamber 7, when the piston is in the normal brakes-off position 
illustrated, through a compensation port 11. Connected to the outer end of 
the boss 9 by a seal 12 is a reservoir 13. A valve assembly is located in 
the boss 9 and retained therein by a circlip 14. The assembly comprises a 
cup-shaped seat member 15 sealed to the boss 9 around its periphery by a 
seal 16 and including a small central hole 17 in its bottom wall and a 
recess in the under surface of the bottom wall, which forms a passage 
connecting the ports 10 and 11 with the hole 17. 
A cap 18 having a hole in its end wall rests on the seat member 15 and 
supports one end of a spring 19, the other end of which acts on a valve 
member 20. The valve member which may be moulded from plastics material 
has a bore in which a ball 21 is held trapped by a transverse pin 22, 
longitudinal grooves being formed in the bore to assist flow of fluid past 
the ball 21. The ball co-operates with the upper end of the valve member 
under the action of pressure developed by fluid flow from the master 
cylinder chamber to the reservoir thereby forming a one-way valve 
controlling flow of fluid through the bore of member 20. The ball is 
sufficiently flow-sensitive to be urged to the closed position at all 
actuation rates and yet can easily move downwardly to permit fluid to flow 
to the master cylinder from the reservoir 13. The bottom surface of valve 
member 20 is flat and is formed with three short feet 23 which are spaced 
apart around its periphery and normally hold this surface at a small 
clearance, for example approximately 0.1 mm, from the bottom wall of the 
seat member 15, so as to define a restricted passage 24 through which 
ports 10 and 11 communicate with the reservoir 13. 
At a normal actuation rate, as the piston 4 moves forward from the position 
illustrated, the volume of the quick-fill chamber 8 decreases and fluid is 
transferred to the pressurising chamber 7 past the seal 5, since the 
pressure is not high enough to open the pressure responsive valve by 
lifting valve member 20 off its seat 15 against the force of the spring 
19. When the pressure in chamber 7 increases to prevent further transfer 
of fluid into this chamber, the valve 20 opens so that the fluid can then 
flow to the reservoir 13 from the quick-fill chamber. In the event of a 
fast actuation due to an operator stamping on the pedal, the small hole 17 
in seat member 15 prevents the valve 20 opening immediately and the fluid 
escaping to reservoir 13 from chamber 8 before sufficient fluid has been 
transferred to the pressurising chamber for the quick-fill operation. 
The quick-fill operation is also obtained at slow actuation rates since the 
restricted passage 24 defined between the confronting surfaces of the seat 
and valve members 15, 20 has sufficient resistance to flow that the 
necessary pressure to transfer fluid past the seal 5 into the pressurising 
chamber 7 is generated in the quick-fill chamber 8. At the same time the 
passage 24 allows the pressure in the quick-fill chamber to decay away so 
that this pressure does not resist brake application and produce a strange 
feel at the pedal. In addition, the passage 24 allows the braking circuit 
to "breathe" via the compensation port 11 during periods of non-use of the 
master cylinder. 
When the piston is moving through the return stroke, the ball 21 of the 
one-way valve moves down to the illustrated position allowing fluid to 
flow back to the quick-fill chamber 8. 
From the above it will be understood that the quick-fill operation is 
achieved independently of the actuation rate, whereby substantially 
uniform pedal travel is obtained for all conditions. There is also an 
added advantage in that when the valve member opens, e.g. during normal 
rate of actuation, the fluid flowing under pressure from the quick-fill 
chamber and through the valve will wash away any deposits from the 
surfaces of the valve and seat members which define the restricted passage 
24, and automatic self-cleaning of this passage 24 is achieved. 
The master cylinder illustrated in FIGS. 2 to 4 comprises a body 51 having 
a stepped bore with larger and smaller diameter portions 52, 53 separated 
by a shoulder. A piston 54 received in the bore has a first end seal 55 
sliding in bore portion 52 and a rear end seal 56 sliding in bore portion 
53. A low volume pressurising chamber 57 is defined in bore portion 52 in 
front of the piston 54, and a larger volume quick-fill chamber 58 is 
defined around the piston between its seals 55, 56. The body 1 has an 
integral boss 59, the interior of which communicates with the quick-fill 
chamber 58 through a port 60 and with the pressure chamber 57, when the 
piston is in the normal brakes off position as shown, through a 
compensation port 61. Connected to the outer end of the boss 59 by a seal 
62 is a reservoir 63. 
The boss 59 defines a valve chamber the bottom wall of which has a central 
spigot 80 through which the passages 60 and 61 open through a common port 
81. An internal peripheral groove 82 in the boss defines an inwardly 
directed shoulder 83 which serves to retain a pre-assembled valve unit 
within the valve chamber. The valve unit comprises a valve mechanism held 
captured within an annular metal cage 89 having a bottom wall which sits 
on the bottom of the valve chamber around the spigot 80, a side wall out 
of which tangs 85 are struck to engage the shoulder 83 and retain the unit 
within the boss 59, and an inwardly directed top flange 86. The valve 
mechanism comprises an annular seal 87 of elastomeric material, e.g. 
rubber which rests on the bottom wall of the cage, a valve member 88 which 
is generally tubular with an external flange 89 whose lower face is 
substantially flat for cooperation with the seal 87 and an internal valve 
seat 90 at its upper end, a ball 91 held captive within the valve member 
by indentations 92 and cooperating with seat 90 to form a one-way valve, 
and a spring 93 acting between the flanges 86, 89 of the cage and valve 
member to urge the flange 89 into sealing contact with the seal 87. 
As may be seen more clearly in FIGS. 2 and 3, the seal 87 has an upwardly 
and inwardly inclined lip 94 on its inner periphery for sealing around the 
spigot 80. On its upper surface the seal has a shallow (e.g. 1 mm) spiral 
groove 95 extending through nearly 360.degree. around the seal axis, the 
groove being open to the inner and outer peripheries at its respective 
ends to communicate respectively with port 81 and with the reservoir 63 
via the valve chamber. The upper surface portions 96 of the seal radially 
inside and outside the groove are chamfered, as seen in FIG. 4, to ensure 
a good sealing contact with the flange 89 of the valve member 88. A series 
of notches 97 are distributed around the outer periphery of the seal and 
serve to prevent undue distortion of the radially inner parts of the seal 
when it is pressed into the cage 84. 
From the foregoing description it will be understood that the valve unit 
can be pre-assembled and correctly mounted on the master cylinder body by 
insertion into the boss 59 until the tangs 85 snap behind the shoulder 83. 
The operation of the master cylinder is essentially the same as that 
described in connection with FIG. 1. When the brake pedal is depressed the 
piston 54 is displaced forwardly and as the volume of chamber 58 decreases 
fluid is transferred from this chamber to chamber 57 for taking up 
clearances in the braking circuit. After the clearances have been taken up 
the pressure increases in chamber 58 and the valve member 88 is lifted 
against the force of spring 93 to allow fluid to pass to the reservoir 63 
from the quick-fill chamber 58. Any residual pressure in the quick-fill 
chamber when the valve member 88 re-engages the seal 87 can decay through 
the restricted passage formed by the spiral groove 95. This restricted 
passage also allows the pressure chamber 57 to breathe, via the 
compensation port 61 during periods of non-use of the brake. On the return 
stroke of the piston, the ball 91 of the one-way valve disengages from its 
seat 90 to allow fluid to flow freely from the reservoir to the quick-fill 
chamber. 
There is no adverse effect on the master cylinder operation if any 
particles of dirt should happen to become trapped between the seal 87 and 
the valve member 88 when they are closed together. Due to the elastomeric 
nature of the seal material any such particles will be pressed into the 
surface of the seal and an effective seal will still be obtained. Thus if 
the next brake application is at a slow rate, the restricted passage 
provided by the spiral groove will still ensure that sufficient liquid is 
transferred to chamber 57 to obtain quick-fill operation, and any danger 
of all the fluid passing to the reservoir from chamber 58, thereby 
increasing the pedal travel, is avoided. 
Various modifications are possible to the described embodiments and will 
occur to those readers skilled in the art. For example, in the master 
cylinder of FIG. 2 it is not necessary to form the groove in the seal and 
it could instead be provided in the valve member.