Chevrolet Light Duty Truck 1973 Service Manual page 277

springs retract all brake shoes and the connecting links
push the wheel cylinder pistons inward, forcing fluid
back to the master cylinder. With the piston bearing
against the retaining ring and the lips of the primary
seals just clear of the compensating ports, excess fluid
which entered through the bleeder holes or was created
by expansion due to increased temperature, now returns
to the reservoirs through the uncovered compensating
ports.
SINGLE PISTON FRONT DISC BRAKES (Fig.
20
)
When fluid is contained in a closed system and pressure
is applied to it, this pressure is exerted equally in all
directions (fig. 4). In the single piston mechanism,
hydraulic pressure acts on two surfaces.
The first, and most obvious, is the piston. The second is
in the opposite direction against the bottom of the bore
of the caliper housing. Since the area of the piston and
bottom of the caliper bore are equal, equal forces are
developed.
Hydraulic force in the caliper bore is exerted against the
piston which is transmitted to the inner brake shoe and
lining assembly and the inner surface of the disc. This
tends to pull the caliper assembly inboard, sliding on the
four rubber bushings. The outer lining, which rests on
the caliper housing, then applies a force on the outer
surface of the disc and together the two linings brake the
car. Since an equal hydraulic force is applied both to the
caliper housing and the piston, the force created against
the outer surface of the disc is the same as the inner.
Since there are equal forces on the linings, no flexing or
distortion of the disc occurs regardless of the severity or
length of application, and lining wear will tend to be
equal.
Note the running clearance of the brake shoe and the
brake disc in Figures 21 and 22. When the brake pedal is
depressed, the piston being in contact with the other side
of the brake shoe, applies force to the inner surface of
the disc. This force causes the caliper to move inboard
until an equal force is applied to the outer disc surface.
The movement that takes place is very slight and would
have to be observed very closely to be seen.
As the brake linings wear, the caliper assembly moves
inboard and fluid fills the area behind the piston, so that
Fig. 20--Disc Brake Operation
BRAKES 5-11
LIGHT DUTY TRUCK SERVICE MANUAL