GMC 1976 ZEO 6083 Maintenance Manual page 221

5- 2 BRAKES
upper end of the secondary shoe away from the an-
chor 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 . This
causes the adjuster lever to pivot on the secondary
shoe forcing the adjuster lever against the adjusting
screw sprocket. If the brake linings are worn enough
to allow the secondary shoe to move the predeter-
mined distance, the adjuster lever will turn the ad-
justing screw sprocket one or two teeth, depending
on lining wear. If the secondary shoe does not move
the predetermined distance, movement of the ad-
juster lever will not be great enough to rotate the
adjusting screw sprocket .
When the brakes are released, the adjusting lever
return spring will move the adjuster lever into the
adjusting position on the sprocket .
An override feature is built into the self-adjusting
brake which allows the secondary shoe to be applied
in reverse in the event the adjusting screw becomes
"frozen" preventing the self-adjuster from operating.
When the vehicle is moving forward and the
brakes are applied, the upper end of the secondary
shoe is forced against the anchor pin due to the self
energizing action of the brakes and the self-adjuster
does not operate.
OPERATION (DISC TYPE)
The significant feature of the single piston caliper
operation is that it is free to slide on the two mount-
ing bolts which thread into the support bracket.
At application of the brakes, the hydraulic pres-
sure behind the 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 inboard
shoe and lining, forcing the lining against the inboard
disc surface. The pressure applied to the bottom of
the piston bore forces the caliper to slide or move
inboard on the mounting bolts. Since the caliper is
one piece, this movement toward the vehicle causes
the outboard section of the caliper to apply pressure
against the back of the outboard shoe and lining
assembly, forcing the lining against the outboard disc
surface . As hydraulic pressure builds up, the shoe
and lining assemblies are pressed against the disc
surfaces with increased force, bring the vehicle to a
stop.
In actual practice, the application and release of
the brake pressure causes a very slight movement of
the piston and caliper. Upon release of the braking
effort, the piston and caliper merely relax into a
released position . In the released position, the shoes
do not retract any appreciable distance from the disc
surfaces .
As the brake lining wears, the piston moves out
of the caliper bore and the caliper repositions itself
on the mounting bolts and equal distance inboard. In
this manner the caliper assembly maintains the in-
board and outboard shoe and lining in the same rela-
tionship with the disc surface throughout the full life
of the lining .
MASTER CYLINDER
DESCRIPTION
The dual master cylinder is designed so that the
front and rear brakes have separate hydraulic sys-
tems . The hydraulic pressure developed in both sys
tems is equal at all times since the front piston is
balanced between the hydraulic pressure in each sys-
tem .
Malfunction in either system has no effect on the
other system but is immediately evident to the driver
because of the additional pedal travel required to
actuate the remaining half of the dual brake system .
Also, a pressure differential sensing switch in the
system actuates a warning light on the instrument
panel.
OPERATION
Two brake fluid reservoirs are cast integrally
with the master cylinder and supply fluid to the areas
ahead of the primary seals through the compensating
ports and between the primary and secondary piston
seals through by-pass ports in the casting.
Within the bore of the master cylinder is the rear
piston assembly, which is composed of the piston,
return spring, retainer, spacer and screw. Also in the
bore is the front piston with a primary and two sec-
ondary piston seals are a front piston return spring
and retainer .
Fluid is directed to the wheels through two hy-
draulic outlets, one for the front brakes and one for
the rear brakes .
In the event the engine stalls, the vacuum cham-
bers within the power cylinder provide adequate
vacuum reserve for two or three brake applications .
If the vacuum check valve is defective or braking has
depleted the vacuum reserve, the driver can still op-
erate the brakes in a conventional manner but more
effort is required due to the loss of power assist .
WHEEL CYLINDER
OPERATION
Each wheel cylinder contains two pistons and