Ford COUGAR 1968 Shop Manual page 18

GROUP
2-Bra
kes
FIG. 7-Dual-Master
Cylinder
brake pedal movement causes the un-
restricted secondary piston to bottom
in the master cylinder bore. Primary
piston movement deplaces 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 unrestricted 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 sys-
tem, actuating the rear brakes.
Oh 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 passage to the outlet
passage, by passing the proportioning
valve. This provides full hydraulic
pressure to the rear brake system.
The increased pedal travel and the
increased pedal effort required to com-
pensate for the loss of the failed
portion of the brake system provides
a warning that'a partial brake system
failure has occured. When the ignition
switch is turned to the S T A R T 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 i f 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. 3).
A mechanically operated electrical
switch is located on the side of the
pressure differential valve assembly.
The inner-end of the.spring loaded
switch plunger contacts the bottom of
a tapered shoulder groove in the
center of the valve (Fig. 3). O-ring
seals are retained in the seal ring
lands of the valve.
Should a failure of the rear brake
system occur, hydraulic fluid pressure
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 to-
ward the low pressure area, or toward
the rear brake system outlet port (Fig.
3). Movement of the differential valve
forces the switch plunger upward over
the tapered shoulder of the valve to
close the switch electrical contacts and
light the dual brake warning lamp.
signalling a brake system 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 operation
of the other brake system.
DISC BRAKE ASSEMBLIES
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 the front and rear
wheels of all others.
A vacuum booster is available as
optional equipment.
The master cylinder converts phys-
ical force from the brake pedal (and
booster i f so equipped) into hydraulic
pressure against the pistons in the
calipers (disc brakes) or in the wheel
cylinders (drum brakes). The pistons
in turn convert hydraulic pressure
back into physical force at the brake
shoes.