Pontiac 1249146 Drum Assembly, Rear Brake

Whenever the disc brake pads are examined for their condition, it is advisable to determine the state of the disc (rotor) surfaces; light ploughing or grooving is permissible, deep cuts or severe troughing is impermissible. On models made prior to 1974, there is only a single deep groove referred to as the 'squeal' groove, and this is not an indication that the groove is worn. If vibration is felt during the operation of the brake pedal, then Disc run out may be the cause. To check for this, there is the use of a dial gauge or feeler blades between the discs and a fixed surface, the runout should not go beyond 0.004 inches. Any variation in thickness at disc thickness more than 0.0005 inch will cause vibration at the time of brake operation. Discs usually have wear limit and refinish thickness dimensions incorporated in them and if a dealer cannot refinish a disc to the stated limit, then a new disc has to be fitted. Also, when a brake drum is pulled for checking lining it is also convenient to inspect for crack, scoring or conditions, out-of-round. An out-of-round drum also induces pedal pulsation when applied and the internal diameter should be checked at least one point using an internal micrometer. A drum can be refinished internally, as long as wear and refinish sizes cast into it are not over sized.

If the scoring is extensive, have the drum turned, but never more than 0.060 in. (1.5mm).

A new disc must be installed if a dealer cannot refinish a disc to meet specified tolerances.

The internal diameter should be measured at several points using an internal micrometer.