Brake Roughness or Chatter - Pedal Pumping

1. Excessive lateral runout of rotor.
2. Rotor excessively out of parallel.

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 more than specifications allow, the disc should be replaced with a new one.
Excessive lateral runout 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 runout, wheel bearings should be adjusted. Be sure to make the adjustment according to the recommendations given in the individual truck chapters.