Pontiac FIREBIRD 1972 Service Manual page 412

BRAKES
-
GENERAL 5-9
COVER RETAINING BAIL RUBBER DIAPHRAM RESERVOIR COVER
\ / /
Fig. 5-2 Typical Master Cylinder
cup, protector, spring and spring retainer; a piston stop
screw and O-ring seal; springs and tube seat retainers
mounted in the two outlet ports provide the operating
components of the two separate hydraulic systems.
The master cylinder front piston supplies the correct fluid
displacement for the front wheel brake requirements. The
rear piston supplies the correct fluid displacement for the
rear wheel brake requirements. The hydraulic pressure
developed in both systems is equal at all times since the
front piston is balanced between the hydraulic pressure in
both systems. If the front system fails, the front piston and
spring bottom against the end of the bore and then the rear
piston develops hydraulic pressure to the rear wheels. If
the rear system fails, the rear piston and spring bottom
against the front piston and then mechanically force the
front piston forward to develop hydraulic pressure to the
front wheels.
The parking brake warning light, a telltale lamp in the
instrument cluster, also operates as the brake hydraulic
system failure warning light. Warning of hydraulic system
irregularities is indicated with the foot brake applied and
the parking brake in the released position. A sliding piston
pressure switch in the junction block which is located on
the frame in drum brake vehicles and in the combination
valve in disc brake vehicles is normally held in the off
position by equal pressures from the front and rear hy-
draulic systems. The sliding piston switch is activated
when a pressure differential is caused by a drop in pressure
covers the brake fluid reservoirs to seal the brake system
from contamination.
Wheel cylinders have corrosion-resistant, double pistons
for even distribution of pressure to each brake shoe. Inter-
nal design rubber boots seal against dust, moisture, and
foreign material.
OPERATION
SELF ADJUSTING BRAKE MECHANISM
The self-adjusting brake mechanism (Fig. 5-3) operates
only when brakes are applied while the car is moving
rearward. This action causes the secondary shoe to move
a pre-determined distance toward the brake drum provid-
ing the brake linings are worn enough to allow this move-
ment.
As the car moves in reverse with brakes applied, the pri-
mary shoe is forced against the anchor pin. At the same
time, hydraulic pressure in the wheel cylinder forces the
upper end of the secondary (rear) shoe away from the
'
anchor pin. As the secondary shoe moves away from the
anchor pin, the upper end of the adjuster lever is prevented
from moving by the actuating link which is attached to the
anchor pin. Since the adjuster lever pivots on the second-
ary shoe, this movement forces the lower end of the ad-
of either the front or rear brake lines. A rubber diaphragm
juster lever against the adjusting screw star wheel.