Dual center-port master cylinder

A dual master cylinder having a bore of uniform diameter extending through an open end thereof and slidably supporting therein a pair of tandem pistons defining a pair of pressurizing chambers. Fuel input is controlled by center-port poppet valves, and the relative disposition of inlet and outlet brake fluid passages are reversed compared to conventional master cylinders to facilitate master cylinder recession into a vacuum booster without requiring long, angled brake fluid passages and to require a shorter and lighter housing. The inlet and outlet passages are positioned so that seals do not slide over fuel passage ports during operation, eliminating seal wear caused thereby and allowing the ports to be larger, thus reducing back pressure and improving compensation when the pistons are returning to, or are in, an unstroked position.

TECHNICAL FIELD 
This invention relates generally to vehicle brake master cylinders and 
specifically to tandem, dual-piston type master cylinders. 
BACKGROUND ART 
For some time, master cylinders of the dual, tandem type have been used in 
automotive vehicles. Such cylinders have primary and secondary 
pressurizing chambers operated by respective primary and secondary pistons 
linearly disposed in a single main bore. 
Vehicle brakes are commonly arranged in circuits, each circuit being 
connected to an outlet of a pressurizing chamber. A typical arrangement 
has front brakes operated by brake fluid from one pressurizing chamber and 
rear brakes operated by brake fluid from the other pressurizing chamber. 
Known types of master cylinders require that the inlet, or replenishing 
port, of each pressurizing chamber from the master cylinder reservoir be 
disposed closer to the open end of the master cylinder housing than is the 
outlet. 
Since this master cylinder has its open end recessed into a vacuum booster, 
no inlet connections can be readily made proximate this end. Accordingly, 
a long, obliquely angled, generally axially extending inlet passage may be 
provided in the master cylinder housing to communicate brake fluid from 
the reservoir to the pressurizing chamber disposed closest to the open end 
of the cylinder. This requires a relatively thick housing wall, adding 
weight and expensive machining to the system. 
Such a system may also use a central control valve including a tappet 
slidably accommodated in a longitudinal bore of each piston and whose 
pedal-side end abuts a stationary bolt that extends transversely through 
the piston bore of the master cylinder and lifts the valve ball from its 
valve seat in the release position. To this end, the valve ball is held in 
a cage that encloses a rubber cushion or plug made of elastic material and 
that can be displaced in opposition to the force of a closure spring, all 
of which adds to the complexity and expense of the system. 
Examples of such a system are shown in U.S. Pat. Nos. 4,979,426; 5,013;096; 
and 5,056,313. As shown in the latter of these, where the advantage of 
recessing the master cylinder in the vacuum booster is given up, the inlet 
passage of the primary piston can be located relatively near the open end 
of the cylinder. 
A further example of the known prior art is shown in FIG. 1 of the drawing, 
which is a sectional representation of a typical, standard, prior art 
master cylinder. The master cylinder is shown including an elongate 
housing having a main bore. The housing also has primary and secondary 
inlet passages that are connected to a brake fluid reservoir. 
Primary and secondary pistons are disposed within the main bore, defining a 
primary pressurizing chamber between the primary and secondary pistons and 
a secondary pressurizing chamber between the secondary piston and a closed 
end of the housing. The pistons are slidable between unstroked and stroked 
positions. Communication is maintained between the reservoir and each 
secondary pressurizing chamber through secondary vent ports during a 
compensation cycle when the pistons are returning to their unstroked 
position. 
In operation, as the pistons begin to move toward the closed end of the 
housing, at least one of the seals on each of the primary and secondary 
pistons slide over the orifices of the primary and secondary vent ports of 
the standard master cylinder. 
To reduce seal wear and increase seal longevity, the diameter of the 
primary and secondary vent ports proximate their intersection with the 
main bore are necessarily small compared to those of the primary and 
secondary inlet passages. The small orifices can result in increased back 
pressure, however, especially when brake fluid is cold. 
While the prior apparatuses function with a certain degree of efficiency, 
none disclose the advantages of the improved master cylinder of the 
present invention as is hereinafter more fully described. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is to provide an improved master 
cylinder requiring no complicated stepped bore and no long, angled 
passages. 
Another object of the present invention is to provide a master cylinder 
having a greater brake fluid flow capability during a compensation cycle, 
when pistons are returning to, or are in, their unstroked positions, than 
known master cylinders. 
Still another object of the present invention is to provide a master 
cylinder requiring fewer components and, therefore, being less expensive 
than known master cylinders. 
Yet another object of the present invention is to provide a master cylinder 
requiring only two inserted subassemblies. 
Another object of the present invention is to provide a master cylinder 
having compensated pressurizing chambers, that is, one providing direct 
brake fluid communication between a brake fluid reservoir and brake 
cylinders or an antilock braking system valve during a compensation cycle 
when the master cylinder pistons are returning to their unstroked 
positions. 
An advantage of the master cylinder of the present invention is that it is 
shorter and lighter than known master cylinders of similar construction 
and function. 
Another advantage of the present invention is that, due to the disposition 
of its inlet and outlet passages, the present master cylinder can be 
readily recessed into the vacuum booster, thereby reducing the overall 
dimensions and weight of that combination. 
Still another advantage of the present invention is that, in not using 
complex current center-port designs, the present master cylinder is less 
expensive than those previously available. 
A feature of the present invention is that, since seals do not slide over 
inlet and outlet passage ports during master cylinder operation, the seals 
can be expected to have a longer functional life. 
In realizing the aforementioned and other objects, advantages and features, 
the dual center-port master cylinder of the present invention includes an 
elongate housing having a main axis. The housing defines therein a 
longitudinally disposed main bore of uniform diameter and concentrically 
disposed along the main axis, and it has a closed end and an open end. 
A secondary inlet passage, which is connectable to a brake fluid reservoir, 
extends obliquely into the main bore at the closed end of the housing. A 
secondary outlet passage intersects the main bore proximate the closed end 
and at substantially right angles to the main axis and to the secondary 
inlet passage. 
The housing also has therein a primary inlet passage, which is also 
connectable to a brake fluid reservoir and which intersects the main bore 
at substantially right,angles at a location farther from the closed end of 
the housing than the secondary outlet passage. A primary outlet passage 
intersects the main bore at substantially right angles at a location 
farther from the closed end of the housing than the primary inlet passage. 
A secondary piston, having a leading end and a trailing end, is disposed 
within the main bore and cooperates with the closed end of the housing to 
define a secondary pressurizing chamber therebetween. The secondary piston 
is slidable between unstroked and stroked positions that respectively 
define maximum and minimum secondary pressurizing chamber volumes. The 
secondary piston sealably isolates the secondary inlet and outlet passages 
from the primary inlet and outlet passages, the primary outlet passage 
being disposed proximate the trailing end of the secondary piston when the 
latter is in its unstroked position. Communication is maintained between 
the secondary inlet and outlet passages during a compensation cycle when 
the secondary piston is returning to, or is in, its unstroked position. 
A primary piston is disposed within the main bore and cooperates with the 
secondary piston to define a primary pressurizing chamber therebetween. 
The primary piston is slidable between unstroked and stroked positions 
that respectively define maximum and minimum primary pressurizing chamber 
volumes. The primary piston sealably isolates the primary inlet and outlet 
passages from the open end of the housing. Communication is maintained 
between the primary inlet and outlet passages during a compensation cycle 
when the primary piston is returning to, or is in, its unstroked position.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 2 of the drawing is a sectional representation of a preferred 
embodiment of a dual center-port master cylinder, generally indicated by 
reference numeral 10. The master cylinder 10 is shown to include an 
elongate housing 12 having a main axis A--A extending longitudinally 
therethrough. A main bore 14 having a uniform diameter is concentrically 
disposed along the main axis A--A within the housing 12. The housing 12 
has a closed end 16 at one end of the main bore 14 and an open end 18 at 
the other end of the main bore 14. 
The housing 12 also has therein a primary inlet passage 20. It is 
connectable to a brake fluid reservoir 80 via a primary inlet brake fluid 
line 84. The primary inlet passage 20 intersects the main bore 14 at 
substantially right angles to the main axis A--A, forming a primary inlet 
passage port 21. A primary outlet passage 22 intersects the main bore 14 
at substantially right angles both to the main axis A--A and to the 
primary inlet passage 20, forming a primary outlet passage port 23. The 
primary outlet passage 22 is connectable to brake cylinders or to an 
antilock braking system valve (neither being shown). 
A secondary inlet passage 24 extends obliquely into the main bore 14 at the 
closed end 16 of the housing 12. The secondary inlet passage is 
connectable to a brake fluid reservoir 80 via a secondary inlet brake 
fluid line 86. A secondary outlet passage 26 intersects the main bore 14 
proximate the closed end 16 and at substantially right angles to the main 
axis A--A, forming a secondary outlet passage port 27. The secondary 
outlet passage 26 is similarly connectable to brake cylinders or to an 
antilock braking system valve (neither being shown). 
A secondary piston 28, having a leading end 31 and a trailing end 33, is 
disposed within the main bore 14 and defines a secondary pressurizing 
chamber 30 between the secondary piston 28 and the closed end 16 of the 
housing 12. The secondary piston 28 is slidable between unstroked and 
stroked positions (FIGS. 2 and 3 respectively) that define maximum and 
minimum secondary pressurizing chamber volumes respectively. A secondary 
piston spring 38 resiliently biases the secondary piston 28 away from the 
closed end 16 of the housing 12. 
The secondary piston 28 is fitted with a pair of seals 56 and 57, typically 
V-block, or lip, seals or their equivalent, disposed in annular recesses 
therein, the lip seal 56 having a higher pressure differential than the 
lip seal 57. The secondary piston 28 sealably isolates the secondary inlet 
and outlet passages, 24 and 26 respectively, from the primary inlet and 
outlet passages, 20 and 22, the primary outlet passage being disposed 
proximate the trailing end of the secondary piston when the latter is in 
its unstroked position 
Medially defined within the secondary piston 28 is a peripheral recess 
cooperating with the main bore 14 to form an annular gallery 32. The 
annular gallery 32 has a sufficient longitudinal dimension to provide 
continuous communication with the primary inlet passage 20 at all 
positions of the secondary piston 28. Also defined within the secondary 
piston 28 is a lateral interior passage 34, in communication with the 
annular gallery 32. 
A longitudinal interior passage 36 extends between the lateral interior 
passage 34 and the trailing end of the secondary piston. Communication is 
maintained by these passages between the primary inlet and outlet 
passages, 20 and 22 respectively, when the secondary piston 28 is in its 
unstroked position. 
A secondary inlet, or popper, valve 40 is slidably supported by the 
secondary piston 28 and is reciprocatable along the main axis A--A between 
an extended and a retracted position (FIGS. 2 and 3 respectively). The 
Secondary inlet valve 40 has a stem 41 defining therein a secondary inlet 
valve retaining pin slot 59. The reciprocating movement of the secondary 
inlet valve 40 is limited by a secondary inlet valve retaining pin 58 
secured to the housing 12 and disposed in the secondary inlet valve 
retaining pin slot 
A retainer piece 68 is disposed within the main bore 14, proximate the 
closed end 16 of the housing 12. The retainer piece 68 defines therein a 
recess 29 and an axially coextensive longitudinal passage 69 into which is 
pressed a valve seat insert 62, which defines a secondary inlet port 25 
therein. The valve seat insert 62 has a flanked portion that resides 
within the retainer piece recess 29. An O-ring seal 88 is seated within 
the counterbore, surrounding, and proud of, the flanged portion of the 
valve seat insert 62. The retainer piece 68 has integral, spaced posts 67 
projecting therefrom to contact the closed end 16 of the housing 12. 
When in its retracted position (FIG. 2), the secondary inlet valve 40 is 
disposed proximate to, but spaced from, the valve seat insert 62. When in 
its extended position (FIG. 3), the secondary inlet valve 40 is 
resiliently biased against the valve seat insert 62 by a secondary inlet 
valve spring 42, which is disposed in the recess 29 defined within a 
retainer piece 68, the O-ring seal 88 providing a fluid-tight seal between 
the secondary inlet valve 40 and the valve seat insert 62. 
A perforate disk 90, defining therein one or more flow passages 91, is also 
received within the retainer piece 68 to provide a stop for the secondary 
piston spring 38. It should be noted that clearance is provided between 
the periphery of the secondary piston inlet valve 40 and the retainer 
piece 68. When the secondary inlet valve 40 is in its retracted position, 
this clearance allows fluid to flow from the secondary inlet passage 24, 
between the retainer piece posts 67, through the valve seat insert 62, 
through the secondary inlet passage port 25, around the secondary inlet 
valve 40, through the perforate disk 90 and into the secondary 
pressurizing chamber 30. 
The trailing end of the secondary piston 28 is constructed similarly to the 
retainer piece 68 and includes a valve seat insert 62 pressed into the 
longitudinal interior passage 36, an O-ring seal 88, and a perforate disk 
90 having at least one flow passage 91. The perforate disk 90 provides a 
stop for the primary piston spring 48. When the primary inlet valve 50 is 
in its retracted position, fluid is allowed to flow from the primary inlet 
passage 20, through the primary inlet passage port 21, through the valve 
seat insert 62, around the primary inlet valve 50, through the perforate 
disk 90 and into the primary pressurizing chamber 46. 
A primary piston 44 is disposed within the main bore 14 and cooperates with 
the secondary piston 28 to define a primary pressurizing chamber 46 
therebetween. The primary piston 44 is slidable between unstroked and 
stroked positions that respectively define maximum and minimum primary 
pressurizing chamber volumes. A primary piston spring 48 resiliently 
biases the primary piston 44 away from the secondary piston 28. 
Fitted in respective annular recesses therein, the primary piston 44 has a 
seal 54, typically an O-ring, and a lip seal 56, typically the same lip 
seal 56 as used on the secondary piston 28. The primary piston 44 sealably 
isolates the primary inlet and outlet passages, 20 and 22 respectively, 
from the open end 18 of the housing 12. Communication is maintained 
between the primary inlet and outlet passages, 20 and 22 respectively, 
during a compensation cycle when the primary piston 44 is returning to, or 
is in, its unstroked position. 
A primary inlet, or popper, valve 50 is slidably supported by the primary 
piston 44 and is reciprocatable along the main axis A--A between a 
retracted and an extended position (FIGS. 2 and 3 respectively). The 
primary inlet valve 50 has a stem 51 defining therein a primary inlet 
valve retaining pin slot 61. The reciprocating movement of the primary 
inlet valve 50 is limited by a primary inlet valve retaining pin 60 
secured to the housing 12 and disposed in the primary inlet valve 
retaining pin slot 61. 
When in its retracted position (FIG. 2), the primary inlet valve 50 is 
disposed proximate to, but spaced from, the valve seat insert 62, which 
defines a longitudinal interior passage port 37 therein. When in its 
extended position (FIG. 3), the primary inlet valve 50 is resiliently 
biased against the valve seat insert 62 by a primary inlet valve spring 
52, which is disposed in a recess 45 defined within the trailing end of 
the secondary piston 28 in a manner as described for the retaining piece 
68, the O-ring seal 88 providing a fluid-tight seal between the primary 
inlet valve 50 and the valve seat insert 62. 
The apparatus disposed within the main bore 14 of the housing 12 
essentially includes primary and secondary master cylinder subassemblies, 
64 and 66 respectively. The primary master cylinder subassembly 64 
includes the primary piston 44, the primary piston spring 48, the primary 
inlet valve 50, the primary inlet valve spring 52 and the primary inlet 
valve retaining pin 60. The secondary master cylinder subassembly 66 
includes the secondary piston 28, the secondary piston spring 38, the 
secondary inlet valve 40, the secondary inlet valve spring 42 and the 
secondary inlet valve retaining pin 58. 
The housing 12 of the master cylinder 10 is affixed to a vacuum booster 70 
by well-known means such as a bolt, or stud, 74 passing from the vacuum 
booster 70 through a flange 72 on the housing 12 and being secured by a 
nut 76. The master cylinder 10 is actuated when an output power member, or 
push rod, 78 exerts force on the primary piston 44. 
In operation, as a force is applied to the primary piston 44 and is 
transmitted through the primary piston spring 48 to the secondary piston 
28, both pistons and both the primary and secondary inlet valves, 50 and 
40 respectively, begin to translate toward the closed end 16 of the 
housing 12. Translation of the secondary inlet valve 40 is halted when it 
sealingly encounters the valve seat insert 62 formed at the secondary 
inlet passage port 25, and the secondary inlet valve spring 42 compresses 
as the secondary piston 28 continues toward its stroked position. 
Similarly, translation of the primary inlet valve 50 is halted when it 
sealingly encounters the valve seat insert 62 formed at the longitudinal 
interior passage port 37, and the primary inlet valve spring 52 compresses 
as the primary piston 44 continues toward its stroked position. The 
primary and secondary piston springs, 48 and 38 respectively, compress 
until these pistons have reached their respective stroked positions. 
With the primary and secondary inlet passages, 20 and 24 respectively, to 
the reservoir 80 closed, pressure in the primary and secondary 
pressurizing chambers, 46 and 30 respectively, increases, pressurizing 
brake fluid in these chambers and in brake fluid lines connecting the 
primary and secondary outlet passages, 22 and 26 respectively, to brake 
cylinders or to an antilock braking system valve (neither being shown). 
When force is removed from the primary piston 44, it and the secondary 
piston 28 are translated to their respective unstroked positions under the 
influence of the residual brake fluid pressure in the primary and 
secondary pressurizing chambers, 46 and 30 respectively, and the influence 
of the primary and secondary piston springs, 48 and 38 respectively. 
In the master cylinder 10 of the present invention, the relative positions 
of the primary inlet and outlet passages, 20 and 22 respectively, have 
been reversed from those of known designs. The relative positions of the 
secondary inlet and outlet passages, 24 and 26 respectively, have also 
been reversed. The primary inlet passage 20 has been positioned proximate 
the midpoint of the secondary piston 28, closer to the closed end 16 of 
the housing 12 than to the open end 18 thereof. 
As a result, the primary piston 44 does not need as many seals and need not 
be as long to provide compensation, that is, communication between the 
reservoir 80 and the primary pressurizing chamber 46. This, in turn, means 
that the master cylinder also need not be as long, therefore saving space, 
weight and expense. Recessing the master cylinder into an associated 
vacuum booster 70, as shown by FIG. 2, is facilitated since that portion 
of the housing 12 proximate the open end 18 thereof is free of brake fluid 
passage connections. 
This configuration also eliminates the requirement for long, angled inlet 
passages and eliminates the requirement for a complicated stepped bore and 
cross pins. It thereby decreases the complexity and cost of the master 
cylinder. Furthermore, since seals do not slide over inlet ports, the 
former can be expected to provide a longer functional life; and the latter 
can be of increased diameter, thereby reducing back pressure and improving 
the response of an associated traction control system. Also, since this 
configuration uses poppet valves for the primary and secondary inlet 
valves, 50 and 40 respectively, expenses associated with more complex and 
costly valves such as those comprising valve balls, cages, springs and the 
like are eliminated. 
While the best mode for carrying out the invention has been described in 
detail, those familiar with the art to which this invention relates will 
recognize various alternative designs and embodiments for practicing the 
invention as defined by the following claims.