TRUCK SERVICE MANUAL
shown in Fig. 9. Brake lining on a bellmouthed brake
drum will make contact only on the inner surface of the
drum. In addition to cutting the braking surface to a
minimum, it will also cause uneven and rapid wear.
Fig. 10 Scored Brake Drum
temperature. As a result, brake fade, slow recovery and
erratic action will be more noticeable.
Fig. 8 Bulged Brake Drum
extremely high temperatures will not only shorten lining
life, but also can cause heat checks and cracks to form
on the inner surfaces of the drums. These conditions will
become progressively worse until finally the drums will
To recondition a brake drum in a lathe, the drum must be
mounted so that it is centered. Use the proper size cone
to provide accurate centering. Turn drum, taking only
Fig. 9 Bell-Mouthed Brake Drum
light cuts and remove just enough material to clean up
drum. Then grind the finish surface if grinder is available
Scored drums are the result of worn lining to the point
or use emery cloth on a straight piece of wood and polish
where drum to shoe contact is made or an accumulation
the drum friction surface.
of small steel particles imbed themselves in the brake
lining (Fig. 10). The steel particles form a tough scale
NOTE: Brake drums that are otherwise in good condition
which is sometimes harder than the drum. As a result, in
can be turned in a lathe.
either case deep grooves are formed in the friction
surface of the drum.
However, it must be remembered that the recommended
rebore limit for brake drums 14 inches diameter and
Brake drum scoring never improves but continually gets
under must not exceed .060" diameter (total cut) and
worse until both the lining and the brake drum are
brake drums with a diameter over 14" may not be
useless. Attempting to reline brakes without turning
increased more than .080" diameter (total cut).
scored brake drums is impractical because the rough
drum surface will quickly destroy new lining and make
Brake drums should be cleaned thoroughly with a steam
effective braking impossible.
cleaner or hot water. Do not use a solvent which leaves
an oily residue. If inspection shows the drums may be
Brake lining in an eccentric or out-round drum cannot
used without remachining, rub the friction surface with
make full contact with the drum, thus resulting in rapid or
fine emery cloth or sandpaper to remove any foreign
uneven lining wear and could even cause brakes to seize
deposits. If the drum has been reconditioned, clean the
Maximum allowable out-of-round or
friction surface with fine emery cloth or sandpaper and
eccentricity should be . 004".
wash. Next, examine very carefully to see that no metal
chips remain in the drum.
If inspection shows that any of the foregoing conditions
exist, the brake drum should be either turned or
BRAKE SHOES AND LINING
replaced. To assure a balanced braking system, always
install turned or new brake drums in pairs on each axle.
The brake shoe is that part of the brake mechanism to
which the brake lining is attached. The brake shoe with
Remember that each time brake drums are turned, less
the lining retards the rotation of the wheel when forced
metal remains to absorb the heat developed by braking
against the drum during brake applications.
action. Brake drums containing less metal will operate at