Chevrolet Light Duty Truck 1973 Service Manual page 279

The anchor pin is the upper pivot point of the brake
shoes. It is located above the wheel cylinder, and is
secured to the top of the flange plate.
Two brake shoes are used, a primary shoe toward the
front of the vehicle, and a secondary shoe toward the
rear. Brake shoe linings are secured to the shoes. The
primary shoe lining is shorter than the secondary shoe
lining.
The shoes are fitted to the anchor pin at the top and
secured by color-coded retracting springs. At the bottom
they fit into grooves at each end of the star wheel
adjuster and are retained in position by a primary to
secondary connecting spring located above the star wheel
adjuster. Each shoe is also attached to the flange plate by
means of a hold-down pin, which is retained in position
by a hold-down spring and retainer cup.
The star wheel assembly is installed under the primary to
secondary shoe connecting spring. A self-adjusting brake
shoe mechanism consisting of a link, actuating lever,
pawl and pawl return spring is also used. The loop end
of the link is attached to the shoe guide plate and the
hooked end to the actuating lever.
Self Energizing Action (Fig. 24)
The brakes are self-energizing. This means that the
brake unit is designed to assist the driver in forcing the
shoes against the drum. To see how this is done, let's
observe a step-by-step sequence of brake action.
When pressure is applied to the brake pedal, fluid is
delivered under pressure from the master cylinder to
each of the wheel cylinders. At each wheel cylinder the
fluid forces the two pistons outward in the bore. This
motion is transmitted from the piston to the shoes by
thrust rods or "struts".
This action forces the shoes outward at the top causing
the shoes to pivot on the adjusting screw assembly (as the
shoes move outward, they also stretch the pull-back
Fig. 24- Brake Operation
springs which are used later to return the shoes to the
rest position).
When the shoes expand outward, they contact the drum
and tend to rotate with the drum. It is this principle that
allows the self-energizing feature to exist. The primary
shoe (the first shoe from the anchor pin in the direction
of normal rotation), rotating with the drum, moves away
from the anchor pin and exerts a rearward force on the
adjusting screw.
At the same time this is taking place the secondary shoe
is rotating upward until the shoe web contacts the anchor
pin.
We can then see that the force applied to the secondary
shoe is the sum of the apply force on the primary shoe
and the force caused by rotation (friction force) of the
primary shoe. This combination of force is the self-
energizing feature, and the increased force it applies to
the brake shoes results in less physical effort required at
the brake pedal.
The increased braking force applied to the secondary
shoe accounts for the fact that the secondary shoe lining
area is usually larger than that of the primary shoe
lining.
When the brakes are applied with the vehicle moving in
reverse, the rear shoe becomes, in effect, the primary
shoe. The self-energizing action now would be applied to
the front shoe, and the rear shoe would move away from
the anchor pin.
Self-Adjusting Action
With the understanding of the operation of the drum
brake system, we can apply the same step-by-step
sequence to see how the self-adjusting mechanism
operates. In the component description given earlier, we
found that the actuating lever assembly of the self-
adjusting mechanism is mounted on the secondary or
rear shoe.
The lever is attached to the web of the secondary shoe by
the hold down pin and spring. The deep dished washer,
used as the hold down spring seat, passes through the
lever and into the shoe web. Thus, the lever is allowed to
pivot about this point. Also, this spring type mount keeps
the pawl end of the lever in firm contact with the teeth
of the adjusting screw star wheel.
Forward Stops (Fig. 25)
During a forward stop, the shoes expand outward and
contact the drum; they rotate with the drum until the
secondary or rear shoe contacts the anchor pin. Thus the
pivot point for the secondary shoe moves only far
enough to place the lining in contact with the drum.
Subsequent motion is then in the direction of the anchor
pin. When the brakes are released, the secondary shoe is
already in contact with the anchor pin, so its only
movement is to pivot on the anchor pin, and follow the
primary shoe to the rest position. Now, let's see what
affect the forward stop has on the self-adjusting
BRAKES 5-13
LIGHT DUTY TRUCK SERVICE MANUAL