Brake Drum Inspection Drum

Pulsations in the brake pedal can usually be corrected by servicing the front disc brakes. Service the rear drum brakes only if the problem persists.

In order to determine if the front or rear brakes are at fault, test drive the vehicle at a low speed in an area away from people and moving traffic, such as a large parking lot. Apply the parking brake and feel for pulsation. The park brake is a push to release style, it may be necessary to push the park brake twice while performing this procedure if the latching mechanism is reached. If a pulsation is not noticed, the fault is probably in the front brakes.

When the brake drums are removed, thoroughly clean and inspect the brake drums for cracks, scores, deep grooves, being out-of-round and taper. Replace a cracked drum; it is unsafe for further use. Do not weld a cracked drum.

Surface Finish

Normal, light scoring of drum surfaces less than 1.5 mm (1/16 inch) deep is not detrimental to brake operation. Smooth out any light scores. Heavy or extensive scoring will cause excessive lining wear, and resurfacing the drum braking surface might be necessary.

If the linings are slightly worn (but still usable) and the drum is grooved, polish the drum with a fine emery cloth; do not refinish it. Eliminating all drum grooves and smoothing the lining ridges would require removing too much metal and lining. If left alone, the grooves and ridges match and satisfactory service can be obtained. If brake linings are to be replaced, refinish a grooved drum. If a grooved drum is used with new lining, it will wear the lining, making proper brake performance difficult to obtain.

Inside Diameter Check

The inside diameter should be checked for conformance to the following specifications, and to determine if refinishing is possible. The drum has a discard dimension cast into it, which is a wear dimension and not a refinish dimension. If the drum does not meet specifications, it should be replaced. Refer to Component Specifications .

Taper Check

An out-of-round or tapered drum prevents accurate brake shoe adjustment, and is likely to cause excessive wear of other brake parts due to its eccentric action. An out-of-round drum can also cause severe and irregular tire tread wear, as well as causing the brake pedal to pulsate.

When a drum exceeds specification limits in taper and/or out-of-round, refinish the drum to true up the braking surface. Out-of-round, as well as taper and wear, can be accurately measured with an inside micrometer and extension rods. When measuring the drum, take measurements at the open and closed edges of the machined surface and at right angles to each other.

Runout Check

A runout check can be done along with taper/out-of-round checks. It can also be done on a brake lathe, using a dial indicator to check the inside of the drum. Runout specifications are 0.15 mm (0.006 inch).

Balance

During manufacturing, weights are used to balance brake drums; do not remove these weights. After refinishing drums, or when maintaining wheel balance is difficult, check the drums for balance. They can be checked on most off-vehicle balancers. If found to be out of balance, replace the drum.

Brake Drum Inspection Lining

Inspect the brake linings at regular intervals (refer to Maintenance Schedule in Maintenance and Lubrication), and any time the wheels are removed (tire rotation, etc.). When any lining thickness is worn to within 0.76 mm (1/32 inch) of the shoe or rivet, replace the shoe and the lining assemblies in axle sets.

GM replacement brake lining material is recommended to maintain the balance between the front and the rear brake performance. Installation of brake lining material having characteristics different from those of the recommended GM replacement parts can change the intended brake balance of this vehicle.