Pontiac Fiero 1988 Service Manual page 223

5-2 BRAKES
GENERAL DESCRIPTION
COMPOSITE MASTER CYLINDER
This vehicle uses a composite master cylinder which has
an aluminum body and a clear nylon reservoir with fluid
level indicators.
The master cylinder uses a "quick take-up" feature in the
rear chamber to reduce excess pedal travel which may
result from increased fluid displacement required to move
the caliper piston. The quick take-up master cylinder
includes a spring loaded ball check valve which holds pres-
sure in the large-diameter rear chamber. When the brake is
first applied, the movement of the rear piston causes fluid to
be displaced forward, past the primary piston primary seal
and into the primary high pressure chamber, which feeds
the rear brakes. At a predetermined pressure, 480-690 kPa
(70-100 psi), the ball unseats and fluid from the large rear
bore is displaced past the ball and into the reservoir. The
primary and secondary high pressure chambers supply
pressure to the rear and front brakes, respectively, in the
conventional manner. When the pedal is released, the
large-bore chamber is filled with fluid by drawing fluid from
the reservoir around the quick take-up lip seal, and also
through a small orifice in the ball seat.
B R A K E FLUID L E V E L INDICATOR
(Figure 1)
The nylon master cylinder reservoir has two windows
which allow the brake fluid level to be checked without
removal of the reservoir cover.
Outward movement of the piston and inward movement
of the caliper automatically compensate for lining wear. As
the linings wear, the increased area behind the piston is
filled with brake fluid from the master cylinder reservoir.
OPERATION O F COMBINATION VALVE
To prevent early rear wheel lock-up under heavy braking
loads, the proportioning section of the combination valve
proportions outlet pressure to the rear brakes after a prede-
termined rear input pressure has been reached.
The valve has a by-pass feature which insures full system
pressure to the rear brakes in the event of a front brake
system failure. Similarly, full front pressure is retained in
the event of a rear brake pressure failure.
B R A K E P R E S S U R E DIFFERENTIAL
WARNING SWITCH
The brake pressure differential warning switch con-
stantly compares brake pressure in both parts of the sys-
tem. The switch will activate the " B R A K E " warning lamp
on the instrument panel in case of a failure in either part.
The combination valve is designed so the switch will stay in
the warning position once a failure has occurred. The lamp
can only be turned off by repairing the failure and applying a
pedal force as required to develop up to 3100 kPa (450 psi)
line pressure.
OPERATION O F DISC B R A K E S
When the brakes are applied, fluid pressure behind the
caliper piston increases. Pressure is exerted equally
against the bottom of the piston and also against the bottom
of the piston bore. The pressure applied to the piston is
transmitted to the inner shoe and lining, forcing the lining
against the inner rotor surface. The pressure applied to the
bottom of the piston bore forces the caliper to slide on the
mounting bolts toward the center of the car. This movement
causes the outer section of the caliper to apply pressure
against the back of the outer shoe and lining assembly,
forcing the lining against the outer rotor surface. As line
pressure builds, the shoe and lining assemblies are
pressed against the rotor surfaces with increased force,
bringing the car to a stop.
Figure 1 Master Cylinder Reservoir Window (Typical)
DIAGNOSIS AND INSPECTION
B R A K E S Y S T E M T E S T I N G
(Figures 2 and 3)
Brakes should be tested on a dry, clean, reasonably
smooth and level roadway. A true test of brake performance
cannot be made if the roadway is wet, greasy or covered
with loose dirt so that all tires do not grip the road equally.
Testing will also be affected if the roadway is crowned
which would throw the weight of the car toward the wheels
on one side. If the roadway is too rough, the wheels will tend
to bounce.
Test brakes at different car speeds with both light and
heavy pedal pressure, avoid locking the brakes and sliding
the tires. Locked brakes and sliding tires do not indicate
brake efficiency, because heavily braked, but turning