Manufacturing tolerance for flatness and parallelism of the brake disc is held to .0005" while lateral run out of the brake disc surfaces cannot exceed .004" total indication reading. Excessive lateral run out of the brake disc will cause a "knocking back" of the pistons, which will create increased pedal travel and vibration when the brakes are applied. The finish of the frictional surfaces must be maintained at 30-50 micro inches.
In view of the foregoing close tolerances, machining of the discs is not recommended by Chevrolet. It has been found that, once a wear pattern has be established, disc brakes are less susceptible to scoring problems than are drum brakes. Disc surface scoring imperfections less than .015 in dept have no effect on disc brake operation.
When the total disc thickness is less than .965 for the 1" thick disc it should be replaced. Disc thickness less than this can permit the shoes to come out of contact with the shoe abutments and cause malfunction.
Below is a Disc Brake Diagnosis chart.
by Chevrolet Service News Volume 40, March 1967, Number 3
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