Are the brake shoes centered on the backing
plate and contacting the anchor pin correctly?
Are all springs installed properly?
Does the automatic adjusting mechanism work?
Are the linings perfectly clean (sand if needed)?
Do I need to bleed the brakes?
Servicing Brake Drums
With the drum removed, inspect the shoes to
determine the condition of the drum. For instance, if
the linings are worn thin on one side, the drums are
likely to be tapered or bell-shaped. Linings with ridges
in their contact surfaces point out the need for
resurfacing (turning) the drum to remove the matching
Resurfacing is needed when the drum is scored,
out-of-round, or worn unevenly. Some shops resurface
a drum anytime the brake linings are replaced, others
only when needed. Drums are resurfaced using a lathe
in the machine shop of an NMCB and at some shore
installations. Commercial brake drum lathes can be
found in some shops. Make sure you know how to
operate the lathe before attempting to resurface a
drum. Using the wrong procedures will damage the
drum and possible deadline the vehicle.
Before resurfacing the drum, check the
specifications that are cast into the drum (fig. 7-34) or
are provided in the maintenance manual. These
specifications tell you the maximum amount of surface
material that can be removed from the drum and still
provide adequate braking. Typically, a brake drum
should not be more than .060 inch oversize. For
esample. a drum that is 9 inches in diameter, when
new, must not be over 9.060 after resurfacing. To
measure brake drum diameter, use a special brake
drum micrometer (fig. 7-35). It will measure drum
Figure 7-34.Example of specification cast into a brake
Figure 7-35.Using a drum micrometer to measure a brake
diameter quickly and accurately. Replace the drum if it
is worn beyond specifications.
For maximum braking efficiency after the drums
have been resurfaced, the arc of the shoes must match
the drums. This means that the linings must be ground
to match the curvature of the drum when it is
resurfaced. There should be a small clearance between
the ends of the lining and the drum. The shoes should
rock slightly when moved in the drum. If the center of
the linings is not touching the drum, the linings should
be arced (ground). Shops equipped with a commercial
brake lathe have a special attachment to perform this
task. If no attachment is available, the shoes can be
installed but the brakes will not become fully effective
until the linings wear enough to match the braking
surface of the drum. Frequent adjustments will be
needed until they wear sufficiently.
SERVICING DISC BRAKES
All disc brake services begin with sight, sound,
and stopping test. The feel of the brake pedal adds a
check on the condition of the hydraulic system.
Stopping the vehicle will indicate whether the
brakes pull in one direction, stop straight, or require
excessive effort to stop. Listening while stopping
permits a fair diagnosis of braking noises, such as
rattles, groans, squeals, or chatters. Visually
inspecting the parts provides valuable information on
the condition of the braking system.
A complete disc brake service typically involves
four major operations, which are as follows:
Replacing worn brake pads
Rebuilding the caliper assembly
Resurfacing the brake discs
Bleeding the system