A cracked brake drum is unsafe for further service. Replace a cracked brake drum. Do not attempt to weld a cracked brake drum. Smooth up any slight scores. Heavy or extensive scoring will cause excessive brake lining wear. Replace the brake drum if any of these conditions are present.

Polish the brake drum with a fine emery cloth. Do not refinish the brake drum if the following part conditions occur:

Do not attempt to eliminate the grooves in the brake drum or to smooth the ridges on the brake lining. This procedure requires removing too much metal and brake lining. Instead, leave the drum alone and the grooves and the ridges will match, creating satisfactory service.

Replace a grooved brake drum when you replace the brake linings. Using a grooved brake drum with new brake linings will wear the brake linings. Worn brake linings result in poor brake performance.

An out-of-round or tapered brake drum can cause the following conditions:

Out-of-round and taper can be accurately measured with an inside micrometer and extension rods. Take these measurements at the open and closed edges of the machined surface and at right angles to each other. Replace the brake drum when the brake drum exceeds the specification limits for taper and/or out-of-round.

Check the following measurements for major service requirements:

1. Measure the braking surface for the maximum allowable diameter at four equally-spaced points near the middle of the braking surface.

An oversized brake drum causes fading or overheated brakes.

2. Replace the brake drum if any diameter is equal to or greater than the maximum discard diameter stamped into the brake drum.
3. Measure the braking surface for out-of-round at four equally-spaced points near the middle of the braking surface.

Out-of-round brake drums cause chatter at the wheels.

4. Replace the brake drum if any diameter readings are more than 0.38 mm (0.015 in) apart.

1. Measure the brake drum diameters at 4 equally-spaced points near the top of the braking surface.
2. Average the 4 measurements in order to obtain an average diameter near the top of the braking surface.

The brake linings will wear unevenly if the brake drum is tapered or barrel shaped.

3. Measure and average the 4 diameters at the middle of the braking surface.
4. Measure and average the 4 diameters at the bottom of the braking surface.
5. Compare the 4 average diameters in order to determine if the brake drum is tapered or barrel shaped.
6. Replace the brake drum if any of the average diameters are more than 0.25 mm (0.010 in) apart.

1. Block the wheels.
2. Start the engine.
3. Charge the air system to the air compressor governor valve cut-out point.
4. Stop the engine.
5. Have an assistant apply and hold the brake pedal.
6. Apply a soap solution around the front air brake chamber clamp and the breather holes.
7. Verify that no bubbles appear.

7.1.	Lightly tap on the clamp and tighten the clamp bolts if bubbles appear.

7.2.	Overhaul or replace the front air brake chamber if the bubbles continue. Refer to the following procedures:

•	Front Air Brake Chamber Overhaul

•	Rear Air Brake Chamber Overhaul

8. Apply the parking brake.
9. Remove the wheel blocks.