2-10
GROUP 2-Brakes
DISC BRAKES
RETURN SPRING
(SECONDARY)
RETAINER
PRIMARY SYSTEM BRAKE
FLUID RESERVOIR
RETURN SPRING (PRIMARY)
BOOT
PUSH ROD
(SECONDARY)
BRAKE OUTLET
PORT
SECONDARY PISTON
ASSEMBLY
(PRIMARY)
BRAKE OUTLET
PORT
PRIMARY PISTON
ASSEMBLY
FIG.
1
— Dual-Master Cylinder-
Typical
master cylinders is porportioned to
fulfill the requirements of each of the
two independent hydraulic brake sys-
tems (Fig. 1).
If a failure of the rear (secondary)
brake system should occur, initial
brake pedal movement causes the un-
restricted secondary piston to bottom
in the master cylinder bore. Primary
piston movement displaces hydraulic
fluid in the primary section of the
dual-master cylinder to actuate the
front brake system.
Should the front (primary) brake
system fail, initial brake pedal move-
ment causes the unrestriced primary
piston to bottom out against the sec-
ondary piston. Continued downward
movement of the brake pedal moves
the secondary piston to displace hy-
draulic fluid in the rear brake system,
actuating the rear brakes. On disc
brake equipped vehicles, the pressure
differential valve will move to the low
pressure area of the front system. This
movement uncovers the rear brake
system outlet passage and provides a
direct passage from the rear inlet pas-
sage to the outlet passage, bypassing
the proportioning valve. This provides
full hydraulic pressure to the rear
brake system.
The increased pedal travel and the
increased pedal effort required to
compensate for the loss of the failed
portion of the brake system provides
a warning that a partial brake system
failure has occurred. When the igni-
tion switch is turned to the start posi-
tion, a dual-brake warning light pro-
vides a visual indication the warning
lamp is functional. When the ignition
switch is turned to the ON or ACC
position, a dual-brake warning light
on the instrument panel also provides
a visual indication if one portion of
the dual-brake system has become
inoperative.
Should a failure of either the front
or rear brake hydraulic system occur,
the hydraulic fluid pressure differen-
tial resulting from the pressure loss
of the failed brake system forces the
valve toward the low pressure area to
illuminate the brake warning light
(Fig. 2).
A mechanically operated electrical
switch is located on the side of the
pressure differential valve assembly
between the front and rear brake sys-
tem inlet ports. The inner-end of the
spring loaded switch plunger con-
tacts the bottom of a tappered shoul-
der groove in the center of the valve
(Fig. 2). O-ring seals are retained in
seal ring lands near each end of the
valve.
Should a failure of the rear brake
H 1545-A
system occur, hydraulic fluid pres-
sure in the rear brake system would
drop.
During brake pedal operation
the fluid pressure build-up of the front
brake system forces the valve to move
toward the low pressure area, or to-
ward the rear brake system outlet
port (Fig. 2). Movement of the dif-
ferential valve forces the switch plun-
ger upward over the tapered shoulder
of the valve to close the switch electri-
cal contacts and light the dual brake
warning lamp, signalling a brake sys-
tem failure.
In the event a front brake system
failure should occur, greater pressure
from the rear brake system during
brake pedal operation forces the valve
forward, moving the switch plunger
upward onto the valve ramp to light
the brake system warning lamp. How-
ever, failure of either the front or
rear system does not impair opera-
tion of the other brake system.
DISC BRAKES
Disc brakes are available as op-
tional equipment for the front wheels.
The hydraulic brake system employs
single anchor, internal expanding and
self-adjusting drum brake assemblies
on the rear wheels of vehicles with
disc brakes, and on front and rear
wheels of all others.
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