Drum Brake System: Testing and Inspection

BRAKE SHOE INSPECTION

Lining Thickness - Inspect the shoe for minimum lining thickness. The lining should be at least 0.76 mm (0.030 in) above the rivets or shoe surface. Inspect the thickness at all rivet locations, top and bottom of shoe, and both sides. Generally the thickness will vary and there will be a low spot or "limiting" location. Always use the minimum measurement to determine if the shoes need to be replaced.

Overheating - Inspect the shoes for signs of overheating:
- Glazed Appearance - The shoes will have a blackened, shiny appearance.
- Discoloration - The shoe webbing will be a bluish-purple color.
- Warping - Lay the shoe on its side on a hard flat surface, if the shoe wobbles it is warped.

If the shoes show any signs of overheating they should be replaced.

NOTE: Prior to replacing the shoes the cause of the overheating should be determined.

Contamination - Inspect the shoes for signs of contamination from brake fluid (leaking from the wheel-cylinder) or axle grease (leaking from the rear axle seals).

NOTE: Leakage from the rear axle seals is often the result of worn axle bearings. As the bearings deteriorate they develop excessive lateral play, which in turn damages the axle seals.

If the shoes show any signs of contamination they should be replaced.

HARDWARE INSPECTION

Springs - Inspect all springs for:
- Bends or damage to coils or shank.
- Loss of tension
- Heat damage (discoloration)




NOTE: Overheated springs lose their tension and could allow the new shoes to drag and wear prematurely.

Replace all springs if any damage is indicated.

Automatic Adjuster - Verify the automatic adjuster moves freely and is not jammed or frozen. The adjuster should be cleaned and lubricated prior to reinstallation.

BACKING-PLATE INSPECTION

Contact Pads - Clean the contact pads thoroughly and verify they are not burred or grooved. Burrs may be removed with very light sanding. If the contact pads are grooved the backing-plate should be replaced.




NOTE: Grooved contact pads may cause the rear brakes to drag or stick. If you attempt to remove the grooves by grinding or filing, the new shoes/linings will not line up correctly with the drum. This may result in grabby or chirping brakes.

The contact pads should be coated with a high temperature silicone lubricant prior to reinstalling the shoes/linings.

Mounting Bolts - Verify the mounting bolts are secure. A loose backing-plate will cause the rear brakes to be very sensitive or grabby.

WHEEL CYLINDER INSPECTION

Leakage - Inspect the dust boots for signs of brake fluid leakage. If any leakage is present the wheel cylinders should be replaced or overhauled. A slight amount of moisture is normal (from initial lubrication of the piston and cylinder).

Mounting Bolts - Verify the wheel-cylinder is firmly attached to the backing-plate.

Bleeder Screw - Prior to reassembly, verify that the bleeder screw is not frozen and will open.