GMC 1976 ZEO 6083 Maintenance Manual page 222

two rubber cups which are held in contact with the
pistons by a central coil spring with cup expanders
to provide a fluid-tight seal . The wheel cylinder cups
are of a special heat resisting rubber . Cups of this
material must have an expander to hold the lips of
the cup out against the wheel cylinder bore . These
cup expanders are crimped on each end of the wheel
cylinder spring . The inlet port for brake fluid is
located between the pistons so that when fluid pres-
sure is applied, both pistons move outward toward
the ends of wheel cylinders. The pistons impart
movement to the brake shoes by means of connecting
links which seat in the pistons and bear against webs
of the shoes. Rubber boots enclose both ends of the
cylinder to exclude foreign matter. A valve for bleed-
ing the brake pipes and wheel cylinder is located
above the inlet port .
COMBINATION VALVE
A combination is incorporated into the brake sys-
tem. It performs three functions; a balance function,
a metering function, and a warning switch .
METERING VALVE FUNCTION
The metering section of the combination valve
operates to "hold off' hydraulic flow (pressure) until
about 130 psi has been built up in the system before
applying the front brakes . The pressure then blends
to full line pressure at approx . 400-600 psi line pres-
sure . There is no flow restriction when the brakes are
released.
BALANCE FUNCTION OR WARNING
SWITCH
The fluid from front and rear systems is sepa-
rated by a hydraulically balanced sealed piston . A
spring loaded switch plunger rides in a groove in the
switch piston . Any pressure differential sufficient to
overcome the switch plunger spring and friction
causes a shift of the piston in the direction of reduced
pressure, causing the switch plunger to ride up out
of its groove and the switch to make contact and
light the warning light. (100-300 psi differential is
required) .
In addition, this piston is designed to hydrauli-
cally recenter itself once the pressure balance is res-
tored (leak is fixed.)
PARKING BRAKE
OPERATION
The parking brake control system, which applies
POWER CYLINDER
BRAKES 5- 3
the four rear brakes, uses a hand operated lever,
cables and brake shoe levers and struts . The front
cable runs from the hand lever along the underbody
to the front equalizer. The intermediate cable then
runs to outside of each frame rail, and back to an
intermediate equalizer. From this point a cable runs
to each of the four brake drums. Each of these cables
connects to the free lower end of a brake shoe lever.
These levers (one in each rear brake shoe assembly)
pivot on the secondary shoes. Struts are mounted
between the brake shoe lever and the primary shoes.
When the parking brake lever is raised, all cables are
put in tension and the rear brake shoes are expanded
against the drums.
The Power Brake Unit is a self-contained hy-
draulic and vacuum unit, utilizing manifold vacuum
and atmospheric pressure for its power.
This unit permits the use of a low brake pedal as
well as less pedal effort than is required with the
conventional (nonpower) hydraulic brake system .
The unit is mounted on the front side of the dash
panel and directly connected to the brake pedal.
A power brake is used with the brake system to
reduce the braking effort required by the driver . A
combined vacuum and hydraulic unit, which utilizes
engine manifold vacuum and atmospheric pressure,
is used to provide power assisted application of vehi-
cle brakes .
The unit is used in conjunction with a conven-
tional brake system . From the master cylinder con-
nection outward to the wheel units, there is no other
change in the brake system .
In addition to the master cylinder connec-
tions, the unit requires a vacuum connection to the
engine intake manifold (through a vacuum check
valve) and a mechanical connection to the brake
pedal . This unit is self-contained .
The vacuum power unit contains the power pis-
ton assembly, which houses the control valve and
reaction mechanism, and the power piston return
spring . The control valve is composed of the air valve
(valve plunger), the floating control valve assembly,
and the push rod. The reaction mechanism consists
of a hydraulic piston reaction plate and a series of
levers . An air filter, air silencer, and filter retainer
are assembled around the valve operating rod filling
the cavity inside the hub of the power piston. The
push rod or valve operating rod, which operates the
air valve, projects out of the end of the power unit
housing through a rubber dust guard. A vacuum
check valve assembly is mounted in the front housing
assembly for connection to the vacuum source .