other troubles require immediate repairs. A complete
drum brake service involves the following:
Removing, cleaning, and inspecting parts from
the backing plate
Replacing brake shoes
Resurfacing brake drums
Replacing or rebuilding wheel cylinders
Lubricating and reassembling brake parts
Readjusting, bleeding, and testing the brakes
Servicing Wheel Cylinders
Normally, faulty wheel cylinders aredetected when
fluid leaks appear or the pistons stick in the cylinders,
preventing brake application. Many shops service the
wheel cylinders anytime the brake linings are replaced.
NCF units require replacement of faulty
wheel cylinders. Rebuilding of wheel
cylinders is NOT authorized.
To rebuild a wheel cylinder, remove the boots, the
pistons, the cups, and the springs. Most wheel
cylinders can be disassembled and rebuilt on the
vehicle. However, many manufacturers recommend
that the wheel cylinder be removed from the backing
plate and serviced on the bench. This makes it easier
too properly clean, inspect, and reassemble. A rebuild
normally involves honing the cylinder and replacing
the cups and boots.
It is important that the cylinder be in good
condition. Inspect the cylinder bore for signs of pitting,
scoring, or scratching. Any sign of pitting, scoring, or
scratching requires cylinder replacement.
A brake cylinder hone is used when honing is
required. With the cylinder hone attached to an electric
drill, lubricate the hone with brake fluid and insert into
the cylinder. Turn the drill on and move the hone back
and forth one time ONLY. The cylinder bore must not
be honed more than 0.003 inch larger than the original
diameter. Replace the cylinder if the scoring cannot be
cleaned out or if the clearance between the bore and
pistons is excessive.
When honing a wheel cylinder, do not let
the hone pull out of the cylinder. The spinning
hone can fly apart causing bodily harm. Wear
After honing, clean the cylinder thoroughly using
clean rags and recommended brake fluid. Make sure
the cylinder is clean and in perfect condition before
reassembly. The slightest bit of grit or roughness can
cause cup leakage.
When reassembling the wheel cylinder, make sure
the new wheel cylinder cups are the same size as the
originals. Cup size is normally printed on the face of the cup.
Lubricate all parts with clean brake fluid and reassemble.
Never allow any grease or oil to contact
the rubber parts or other internal components.
Grease or oil will cause the rubber parts to
swell. which will lead to brake failure.
Replacing Brake Shoe Linings
To replace the brake shoes, first remove the wheel
and brake drum. With the drum removed, note how the
springs and retainers are installed before attempting to
remove the shoes from the backing plate. This will
assist you during reassembly.
If hydraulic brakes are being repaired, install a
wheel cylinder clamp (fig. 7-29) to prevent the pistons
from coming out of the wheel cylinder. Next, remove
the retracting springs with brake spring pliers or a
removal and installation tool (fig. 7-30). The brake
Figure 7-29.Wheel cylinder clamp in use.
Figure 7-30.Removal of the brake return springs.