Drum brakes are common on older automobiles, and on lower-cost vehicles they are still frequently used on the rear wheels. While not as effective or reliable as modern disc brakes, drum brakes have the advantage of being less expensive and lighter, and when used in the rear position they do not seriously impair the braking function. The parking brake mechanism can also be incorporated into a drum brake, alleviating the need for a separate parking brake mechanism. Unfortunately, the mechanical complexity of drum brakes leaves plenty of opportunity for problems, some of which can cause the brake to stick.
Park the automobile on a flat, level surface. Do not set the emergency brake. Put an automatic transmission in "park," or a manual transmission in first or reverse gear.
Set the parking brake. Typically it should take six to 10 clicks of the pedal or lever to set the brake, and the resistance of the pedal or lever should increase the further it is moved. If the parking brake is adjusted too tight then only a couple of clicks will be required to set the brake. If the trailing brake shoe in the drum is sticking, the parking brake lever or pedal will feel slack right up until the parking brake engages.
Release the parking brake. Partly loosen the lug nuts on both rear wheels. Jack up the rear of the automobile and rest it securely on axle stands. Remove both rear wheels.
Remove the drums on both rear wheels. Some drums are secured by two or four bolts, some by two screws, and others by nothing at all. Remove any retaining bolts or screws and pull the drums off. If the drum is seized, tap it firmly with a mallet on the front shoulder to loosen. Do not hit the rim of the drum where it meets the backing plate. Also, some drums have a hole near the bottom of the drum that allows access to the adjuster mechanism. If the drum is sticking, reach through the hole with a small screw driver and turn the adjuster wheel to relieve tension on the brake shoes.
Carefully examine the brake mechanisms on both wheels. There are two return springs connecting the front and rear brake shoes, and if either of these is broken or weakened, the shoes will not retract properly and they will drag on the drum.
Hold the adjuster lever off of the adjuster wheel and turn the wheel back and forth. If the adjuster wheel is at the end of its travel it will force the shoes to drag on the drum. A seized adjuster wheel may be holding the shoes out against the drum.
Look for signs of corrosion or accumulations of dust and dirt. Heavy deposits where the shoes contact the backing plate, or around the pivot points for the adjuster lever, the operating lever, and the brake retaining pins can seize the brake mechanism and prevent the shoes from retracting normally.
Observe the brake cylinder operation while an assistant starts the automobile and gently pushes the brake pedal a few times. Do not pump the brakes. Allow about five seconds between each push on the brake pedal. The cylinder piston should extend when the pedal is pushed and retract when the pedal is released. A defective cylinder can remain extended and hold the shoes against the drum.
Observe the brake operation while an assistant sets and releases the parking brake. Note that the parking brake will not actually set with the drum released, so take care not to move the parking brake lever or pedal too far. The rear brake shoe should move out when the parking brake is set, and move back when the parking brake is released. If the parking brake cable is rusted, kinked, or otherwise damaged, the rear brake shoe will not return when the parking brake is released, causing the rear shoe to drag on the drum.
- Do not breathe brake dust.
- Be careful when working around springs under tension as they can suddenly snap and cause injury.