Check the brake drum for cracks, heat cracks. heat
checks, hard spots, scoring, or worn beyond
specifications. Damaged drums may be machined
(turned) as long as they still meet the manufacturers
specifications. Badly damaged or worn drums must be
To check the wheel cylinder for leakage, pull back
the cylinder boots. If the boot is full of fluid, the wheel
cylinder should be rebuilt or replaced. Also, check the
return springs and the automatic adjusting mechanism.
SERVICING THE MASTER CYLINDER
When major brake service is being performed, the
master cylinder is to be inspected for proper operation.
A faulty master cylinder usually leaks externally out
the rear piston or leaks internally. You are able to
detect external brake fluid leaks by checking the
master cylinder boot for fluid or dampness on the
firewall. When the leak is internal. the brake pedal will
slowly move to the floor. Inoperative valves in the
master cylinder are also a reason for service.
To remove the master cylinder, disconnect the
brakes lines from the master cylinder using tubing
wrenches. With the brake lines disconnected, unbolt
the master cylinder from the brake booster or firewall.
In some cases. the pushrod must be disconnected from
the brake pedal.
Many shops, however, simply, replace a bad master
cylinder with a factory rebuild or a new one. A
replacement master cylinder is normally cheaper than
the labor cost and parts for an in-shop rebuild.
NCF units require replacement of faulty
master cylinders. Rebuilding of master
cylinders is NOT authorized.
To rebuild a master cylinder, drain the fluid from
the reservoir. Disassemble the master cylinder
following the instructions in the manufacturers
service manual. After disassembly, clean the parts in
brake fluid or a recommended cleaner.
Do NOT clean the hydraulic parts of the brake
system with conventional parts cleaners. They can
destroy the rubber cups in the brake system. Only use
brake fluid or a manufacturers suggested cleaner
(denatured alcohol. for example).
If the cylinder is not pitted, scored, or corroded
badly, it may be honed using a cylinder hone. When the
cylinder is honed, the hone is ran ONLY once in and
out. After honing, measure the piston-to-cylinder
clearance, using a telescoping gauge and an outside
micrometer or a narrow (1/8" to 1/4" wide) 0.006"
feeler gauge. When a feeler gauge is used, if the gauge
can be inserted between the cylinder wall and the
piston, the master cylinder must be replaced. The
cylinder must NOT be tapered or worn beyond the
manufacturers specifications. Replace the master
cylinder if the cylinder is not in perfect condition after
Blow-dry all parts with low-pressure compressed
air. Blow out the ports and check for obstructions.
Lubricate all parts with the recommended brake fluid
and assemble the master cylinder, using the
manufacturers service manual.
After the master cylinder is reassembled, it is good
practice to bench bleed a new or rebuilt master cylinder
before installation on the vehicle. A master cylinder is
bled to remove air from the inside of the cylinder.
Bench bleeding procedures are as follows:
Mount the master cylinder in a vise
Install short sections of brake line and bend them
back into each reservoir
Fill the reservoir with approved brake fluid
Pump the piston in and out by hand until air
bubbles no longer form in the fluid
Remove the brake lines and install the reservoir
Once the master cylinder has been bench bled. it is
ready to be reinstalled on the vehicle. Bolt the master
cylinder to the booster or firewall. Check the
adjustment of the pushrod if there is a means of
adjustment provided. Without cross threading the
fittings, screw the brake lines into the master cylinder,
and lightly snug the fittings. Then bleed (remove air
from) the system. Tighten the brake line fittings. Refill
the reservoir to the proper level and check brake pedal
fall. Last but not least, test the vehicle.
SERVICING DRUM BRAKES
You should understand the most important
methods for servicing a drum brake. However. specific
procedures vary and you should always consult the
manufacturers service manual. Brake service is
required anytime you find faulty brake components. A
leaking wheel cylinder, worn linings, scored drum, or