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ASE P2: Brakes


11/9/2012
By Larry Carley

For safe driving, brake pads must be replaced when they are worn down to minimum thickness specifications.
 
Disc brake pads are one of the most commonly replaced brake parts. Pad wear depends on the friction material, miles driven, vehicle weight, the swept area of the brakes, the type of driving and the driver’s braking style. Front pads typically wear two or three times faster than rear pads, but on some vehicles with electronic brake proportioning, the rear pads may actually wear at a faster rate.

Because of these variables, pad life can vary a great deal, from as little as 30,000 miles or less up to 70,000 miles or more. For safe driving, brake pads must be replaced when they are worn down to minimum thickness specifications. Pads must also be replaced if contaminated with brake fluid or grease. Pads also may be replaced to solve a noise problem or to improve braking performance.

Brake friction materials include nonasbestos organic (NAO), ceramic, low-metallic and semi-metallic. Replacement pads should be the same material (or better) then the original. Some pad sets use different friction materials on inner and outer pads to enhance braking performance.

Other brake parts that may be needed when pads are replaced include new disc brake rotors, calipers, brake hoses, steel brake lines, brake fluid (DOt 3 or DOT 4) and brake lubricant (for caliper mounts and bushings).

The most common reasons for replacing calipers is leaks and sticking. Fluid leaks are dangerous and can lead to brake failure. The fluid also can contaminate the pads causing the brakes to pull unevenly to one side. A caliper that sticks may cause the brakes to drag or to pull to one side when braking. Replacing high-mileage calipers is recommended to reduce the risk of leaks and sticking. “Loaded” calipers come complete with new pads and hardware for easy installation.

Brake rotors can be resurfaced, but must be replaced if worn down to minimum thickness specifications. Rotors should also be replaced if they are cracked, severely corroded, have hard spots or are causing pedal pulsations when braking. Some rotors are directional (the cooling fins must rotate a certain direction for proper cooling). Most rotors are a one-piece iron casting but some are a composite construction with a stamped steel center hat section. Generally, rotors should be replaced with the same type as the original, but one-piece castings are available as a replacement option for many applications originally equipped with composite rotors.

Drum brakes are still used for the rear brakes on many cars and trucks. Drums must be replaced if the inside diameter is worn beyond maximum specifications, or the drum is cracked, out-of-round, has hard spots or is deformed (ball mouthed). Wheel cylinders must be replaced if leaking or sticking. Rear brake shoes must be replaced if worn to minimum thickness specifications or contaminated with brake fluid or grease. New drum brake hardware is also recommended.














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