Service

Fig. 2 Checking Rotor For Lateral Run-out:

Fig. 3 Checking Rotor Parallelism (thickness Variation):

Servicing of disc brakes is extremely critical due to the close tolerances required in machining the brake disc to insure proper brake operation.
The maintenance of these close controls of the shape of the rubbing surfaces is necessary to prevent brake roughness. In addition, the surface finish must be non-directional and maintained at a micro inch finish. This close control of the rubbing surface finish is necessary to avoid pulls and erratic performance and promote long lining life and equal lining wear of both left and right brakes.
In light of the foregoing remarks, refinishing of the rubbing surfaces should not be attempted unless precision equipment, capable of measuring in micro inches (millionths of an inch) is available.
To check lateral run-out of a disc, mount a dial indicator on a convenient part (steering knuckle, tie rod, disc brake caliper housing) so that the plunger of the dial indicator contacts the disc at a point one inch from the outer edge, Fig. 2. If the total indicated run-out exceeds specifications, install a new disc.
To check parallelism (thickness variation), mount dial indicators, Fig. 3, so the plunger contacts rotor approximately 1 inch from outer edge. If parallelism exceeds specifications, replace rotor.


BRAKE ROUGHNESS

The most common cause of brake chatter on disc brakes is a variation in thickness of the disc. If roughness or vibration is encountered during highway operation or if pedal pumping is experienced at low speeds, the disc may have excessive thickness variation. To check for this condition, measure the disc at 12 points with a micrometer at a radius approximately one inch from edge of disc. If thickness measurements vary by more than .0005 inch, the disc should be replaced with a new one.
Excessive lateral run-out of braking disc may cause a ``knocking back'' of the pistons, possibly creating increased pedal travel and vibration when brakes are applied.
Before checking the run-out, wheel bearings should be adjusted. The readjustment is very important and will be required at the completion of the test to prevent bearing failure. Be sure to make the adjustment according to the recommendations given under ``Front Wheel Bearings, Adjust''.