new seal in the cylinder bore groove. Work the
seal into its groove with your fingers. Install the
new boot in its groove. Coat the piston with
more brake fluid. Spread the boot with your
fingers and slide the piston into the cylinder.
The caliper can now be reinstalled on the
Carefully follow the procedures given in the
manufacturers service and repair manuals for specific
details when removing, repairing, and reinstalling disc
brake caliper assemblies.
Brake Disc (Rotor) Service
It is important to check the condition of the brake
disc when servicing the brake system. Vehicle
manufacturers provide specifications for minimum
disc thickness and maximum disc runout. The disc
must also be checked for scoring, cracking, and heat
checking. Disc resurfacing is required to correct
runout, thickness variation, or scoring.
MEASURING DISC THICKNESS.To
measure disc thickness, use an outside micrometer.
Disc thickness is measured across the two friction
surfaces in several locations. Variation in disc
t h i c k n e s s i n d i c a t e s w e a r . C o m p a r e y o u r
measurements to the manufacturers specifications.
Minimum disc thickness will sometimes be
printed on the side of the disc (fig. 7-36). If not, refer to
the manufacturers service manual or a brake
specification chart. If disc thickness is under
specifications. replace the disc, because a thin disc
cannot dissipate heat properly and may warp or fail
BRAKE DISC RUNOUT.The amount of
side-to-side movement, measured near the outer
friction surface of the disc, is known as brake disc
runout. Runout is measured using a dial indicator.
Figure 7-36.Example of minimum thickness specification
cast into a brake disc.
Using a magnetic base, attach the dial indicator to the
hub. Position the dial indicator so it touches the face of
the disc. Rotate the disc by hand and read the indicator.
Compare the indicator reading to factory
specifications. Typically, disc runout should not
exceed .004 inch. If runout is beyond specifications,
resurface the disc to its true friction surface.
RESURFACING A BRAKE DISC.When a
disc is in good condition, most manufacturers do NOT
recommend disc resurfacing. Disc resurfacing is done
when absolutely necessary.
When using a brake lathe to resurface a brake disc,
you use the appropriate spacers and cones to position
the disc on the arbor of the machine. Wrap a spring or
rubber damper around the disc to prevent vibration.
Follow the directions provided with the brake lathe.
Do not attempt to operate a brake lathe
without first obtaining proper training.
Damage to the machine or injury to the
operator can occur as a result of incorrect
Only take off enough metal to true the disc. Then
without touching the machined surfaces with your
fingers, remove the disc. This prevents body oil from
penetrating the machined surfaces. Check the disc for
thickness and reinstall on the vehicle.
BRAKE SYSTEM BLEEDING
Brake system bleeding is the use of fluid pressure
to force air from the system. The brake system must be
free of air to function properly. Air in the system will
compress, causing a springy or spongy brake pedal.
Air may enter the system any time a hydraulic
component (wheel cylinder, master cylinder, hose, or
brake line) is disconnected or removed. There are two
methods of bleeding brakesmanual bleeding and
Manual bleeding uses master cylinder pressure to
force fluid and trapped air out of the system. To bleed
the system, proceed as follows:
Fill the master cylinder reservoir with brake
fluid to 1/4 inch from the top, and keep it full
during bleeding operations.