B
90
Repair and Service Manual
MECHANICAL BRAKES
Read all of SAFETY and this section before attempting any procedure. Pay particular attention to Notices, Cautions, Warnings and Dangers.
699327
BRAKE SYSTEM OVERVIEW
General Description
This vehicle is equipped with a mechanically activated
rear drum brake system. The brake system consists of a
service brake and parking brake pedal, parking brake
release linkage, equalizer link, brake cables, and self
adjusting wheel brake assemblies (Ref Fig. 1).
Although the brake system is similar to an automobile, it
is
different in important ways
. The system operates in
a very severe environment. Fertilizer, dust, grass clip-
pings, tree roots, and other objects can cause corrosion
and physical damage to the brake components. Unlike
automotive hydraulic brakes, mechanical brakes depend
on the travel of the brake cables to move the brake
shoes against the brake drums. The travel of the brake
cables is governed by the brake pedal. If the cables can-
not travel far enough to absorb the slack (free travel) in
the system and still apply the shoes to the drums, the
braking effort at the wheel brake will not be adequate.
The self adjusting mechanism in the wheel brakes
requires enough cable travel at the wheel brake to work
reliably. When the brake is released, there must be slack
in the system so the brakes will release fully and the
adjusters will function. Free pedal travel, pedal force,
shoe to drum clearance, and braking capability are
closely related. It is very important to maintain the
proper relationships to assure braking performance and
the best wheel brake adjustment.
How the Service Brake Works
Pressing the service brake pedal pulls the equalizer link,
which is connected to the brake cables (Ref Fig. 1). The
first part of the pedal travel removes slack from the sys-
tem. Continued motion of the brake pedal pulls both the
left and right brake cables. Each brake cable pulls a
brake lever which pushes the rear brake shoe against
the brake drum. When the rear shoe contacts the brake
drum, it can no longer move rearward. Additional pedal
(and cable) travel causes the actuator bracket (moving
anchor) to move and applies force to the front shoe,
pushing it against the brake drum. The force applied to
the front and rear shoes is approximately equal. As the
shoes contact the moving brake drum, the shoes try to
move in the direction of drum rotation. This movement
results in the typical brake shoe wear patterns.
Equalizer Link
The equalizer link balances braking between the driver
and passenger sides of the vehicle. Variations in wheel
brake adjustment, cable friction and manufacturing toler-
ances may cause the equalizer to be slightly misaligned.
This misalignment is normal.
Automatic Adjuster Mechanism
Never manually adjust the brakes at
the star wheel. Doing so will cause
permanent damage to the adjuster
mechanism and result in a gradual
loss of brakes.
The wheel brakes are equipped with an automatic
adjuster mechanism that is designed to compensate for
brake shoe wear and eliminate the need for manual
brake shoe adjustment. Do not manually adjust the
brakes by prying back the adjuster arm and turning the
star wheel. Permanent damage to the adjuster will
result.
Adjustment takes place only when the brake is fully
applied and released
while the vehicle is moving
.
When the brake pedal is applied, the brake lever moves
toward the front of the vehicle (A) (Ref Fig. 2).
The other end of the brake lever moves to the rear of the
vehicle (B) where it contacts the brake adjuster arm,
causing it to move.
The brake adjuster arm moves away from the star-wheel
(C). The amount of adjuster travel is limited by the
amount of brake shoe travel required to contact the
brake drum. The amount of travel increases as the
brake shoe lining wears.
When the brake pedal is released, the adjuster spring
retracts the brake adjuster arm which contacts the star-
wheel. Note that adjustment only takes place when the
brake pedal is released while the vehicle is in motion.
If the shoes have worn far enough to permit the brake
adjuster arm to contact the next tooth of the star-wheel,
the star wheel will be advanced by the tension applied to
the adjuster arm by the adjuster spring.
Fig. 2 Automatic Adjuster Mechanism
A
C
Brake Lever
Brake Adjuster Arm
Star Wheel
Adjusting Screw
Adjuster Spring
Brake
Shoe
B
PRELIMINARY
