Chevrolet Light Duty Truck 1973, Service Manual

Page: 276 / 979

5 -1 0 BRAKES
of the brakes. Wear in the pedal linkage, pivot pins, or
bushings, can cause loss of pedal or frequent need for
brake adjustment. Pedal free play is the free travel of
the pedal before any movement of the master cylinder
piston occurs. Too little free play can cause brakes to
drag. Too much free play may result in a low pedal. The
free play at the brake pedal pad should be l / 16 to 1/4
inch for the standard height pedal.
OPERATION OF BRAKE
MECHANISM
When the brakes are fully released, the master cylinder
pistons are held against the push rod retainer, and the
primary seals are held just clear of the compensating
C O N T A C T IS MADE
Fig. 18-Stoplamp Switch (Typical)
Fig. 19-B rake Pedal (Typical)
ports by the piston springs. The pressure chambers are
filled with fluid at atmospheric pressure due to the open
compensating ports and the flexible reservoir diaphragm.
When the brake pedal is depressed to apply the brakes,
the push rod forces the master cylinder pistons and
primary seals forward. As this movement starts, the lips
of the primary seals cover the compensating ports to
prevent escape of fluid into the reservoirs. Continued
movement of the pistons builds pressure in the pressure
chambers and fluid is then forced through the lines
leading to the wheel cylinders. Fluid forced into the
wheel cylinders between the cups and pistons causes the
pistons and connecting links to move outward and force
the brake shoes into contact with the drums.
Movement of all brake shoes into contact with drums is
accomplished with very light pedal pressure. Since
pressure is equal in all parts of the hydraulic system,
effective braking pressure cannot be applied to any one
drum until all of the shoes are in contact with their
respective drums; therefore, the system is self-equalizing.
When the brake pedal is released, the master cylinder
springs force the pedal back until the push rod contacts
the retaining ring in the master cylinder. This spring
also forces the pistons and primary seals to follow the
push rod.
At the start of a fast release, the pistons move faster
than the fluid can follow in returning from the lines and
wheel cylinders; therefore, a partial vacuum is
momentarily created in the pressure chamber. Fluid
supplied through the breather ports is then drawn
through the bleeder holes in piston heads and past the
primary seals to keep the pressure chamber filled.
As pressure drops in the master cylinder, the shoe
LIGHT DUTY TRUCK SERVICE MANUAL