Chevrolet Light Duty Truck 1973 Service Manual page 280

5 -1 4 BRAKES
PR IM AR Y S H O E
M O V E S A W A Y
FR O M A N C H O R PIN
Fig. 25--Reaction During Forward Stop
mechanism. First, the small amount of movement from
the rest position until the shoes contact the drum, is
permitted without activating the adjusting mechanism by
the slack inherent to this linkage design. Since the
rotation of the secondary or rear shoe is toward the
anchor pirr, the effective distance from the pin to the
actuating lever is not increased; thus, no adjustment
occurs. When the brakes are released, the linkage relaxes
with any small degree of motion absorbed by the linkage
slack.
Reverse Stops (Fig. 26)
During a reverse stop, the shoes expand outward to
contact the drum, and then rotate with the drum until
the primary or front shoe contacts the anchor pin. When
there is sufficient clearance between the lining and the
drum, this rotation increases the distance from the
anchor pin to the secondary or rear shoe hold down pin.
Since the wire link between the anchor pin and actuating
lever is a fixed length, the movement of the lever pivot
point causes the top of the actuating lever to be pulled
inward. As the lever pivots on the hold down spring cup,
the pawl end rocks down on the adjusting screw star
wheel. When the pawl turns the star wheel, it increases
the length of the adjusting screw in much the same
manner as a service technician using a brake adjusting
tool.
As the lever rocks down to turn the star wheel, it also
moves outboard to follow the contour of the star wheel.
At the same time, the downward motion compresses the
lever return spring. When the brakes are released, the
pull-back springs return the shoes to their normal
position, the lever return spring raises the pawl end of
the lever back to its normal position. The pawl slips back
over the teeth of the star wheel and takes a new "bite"
on another notch.
When there is only a slight clearance between the shoes
and drums, such as the condition when the brakes are
near to proper adjustment, the shoes rotate only a small
amount before the shoes contact the anchor pin. This
slight amount of movement is not enough to cause the
actuating lever to advance the star wheel.
Over-Ride Mechanism (Fig. 27)
All of the standard equipment self-adjusting mechanisms
contain a built-in safety device known as the "over-ride
pivot plate and spring". This pivot plate and spring are
mounted on the upper end of the actuating lever, acting
as a semi-rigid connection between the actuating lever
and the wire link. The over-ride mechanism has one
major function; to prevent damage to the linkage when,
for some reason, the linkage cannot move as dictated by
shoe rotation. Two common examples of this condition
occurs when; the adjusting screw binds, or the brakes are
severely out of adjustment.
If the adjusting screw threads should bind, the self-
adjusting mechanism would attempt to continue
operating. This would result in damaged parts, or
possible reduced brake application on the affected wheel.
When this occurs, the actuating lever remains stationary,
and the over-ride pivot plate is pulled by the wire link.
This motion is absorbed by stretching the over-ride
spring. With the adjusting motion dampened by the
spring, the component parts remain undamaged.
NOTE: One symptom of a bound adjusting
screw is "pulling brakes". Inspect the screw
when a complaint of brake pull is reported.
The second purpose of the over-ride plate is to prevent
excessive actuating lever travel. This condition could
occur if the brake adjustment were extremely loose.
Should this condition occur, the motion of the actuating
lever could allow the pawl end to rock down until it
contacted the secondary shoe. Here again, without the
over-ride spring, linkage damage or reduced brake
efficiency could result.
Equally important to proper brake adjustment is the
prevention of adjusting screw "back-off". The self-
energizing force transmitted by the adjusting screw tends
to compress the screw length. We can thus see the
REVERSE
FIXED LEN G TH
O F W IRE LIN K
C AUSES LEVER
T O P IV O T
Fig. 26--Reaction During Reverse Stop
S E C O N D A R Y SH O E
M O V E S A W A Y FR O M
A N C H O R PIN
LIGHT DUTY TRUCK SERVICE MANUAL