Old Cars Restoration: Bedding your brakes
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The key to a successful brake system restoration is to ensure that designed braking surface exists. That means the full extent of the brake shoes are in contact with the drum, and that brake pads fully contact the rotor when applied.
Preparations
For drum brakes, the inside dimension of the drum should be consistent with the factory-specified inside diameter after it is machined to provide a smooth surface free of grooves and high spots. If the resulting diameter is larger than specified, the drum should be replaced. In days long ago, it was common to adjust the shoe curvature to fit the enlarged drum. But few shops are now equipped and have the personnel with the necessary experience. Secondly, the shoes need to be checked to determine if they are centered in the drum when adjusted. If not, the shoe pivot point needs to be adjusted.
For disc brakes, there are similar preparation steps. The rotors need to be machined to remove any flaws and provide smooth, parallel surfaces. It is important to also remove any rust or debris from the hub mounting surface. Finally, the rotor should be checked with a dial indicator for run-out (the amount of movement when the rotor rotates). If the run-out exceeds the factory specification, a new rotor should be installed.
Brake Bedding
Brake bedding involves heating the brake shoe and drum, or rotor and pad, through braking in order to transfer a thin, uniform layer of braking material to the drum or rotor. Properly bedded brakes will ensure the maximum designed braking action, and smooth braking when the brakes are applied.
Too little heat during bedding prevents the transfer of braking material to the metal surface, and too much heat can create uneven deposits, because the brake material breaks down and adheres to the metal surface. Such deposits can cause uneven contact of the braking surfaces (grip-slip) under pressure. The resulting vibrations will telegraph through the steering wheel. Sometimes referred to as “braking shimmy,” the condition reduces braking performance. With disc brakes, braking shimmy can be misdiagnosed as warped rotors.
Generally, brake bedding involves driving a car to a specific speed, applying the brakes to bring the car almost to a stop, then resuming driving to cool the brakes. This operation is repeated several times. With each succeeding cycle, vehicle speeds are increased.
The makers of many disc brake pads will provide specific procedures for brake bedding to ensure that the correct amount of heat is supplied consistent with the composition of the braking material. Generally, the replacement brake shoes commonly available do not provide such instructions.
For those situations where no specific bedding procedure is provided by the pad or shoe manufacturer, the following is suggested. As should become clear, the bedding procedure must be done in an area where accelerating, braking and coasting can be safely accomplished.
1. Make 6 to 10 stops from 30 to 35 mph using moderate pedal pressure. Do not come to a complete stop; gently accelerate to the testing speed over some distance, allowing for some cooling. Then, repeat the braking, accelerating and cooling until all repetitions are complete.
2. Then, make 2 to 3 hard stops from 40 to 50 mph. Do not come to a complete stop; gently accelerate to the testing speed over some distance, allowing for some cooling. Then, repeat the braking, accelerating and cooling until all repetitions are complete.
3. Do not drag the brakes during the first two steps.
4. Finally, proceed to a safe space, where the car can be gently stopped, and the brakes can completely cool. Do not apply the parking brake.
After the bedding procedure is complete, avoid heavy braking during the first 400 to 500 miles.
William C. “Bill” Anderson, P.E., has been involved with the automotive hobby for more than 50 years with experience ranging from hot rods, to sports cars, to sports car racing, and to restoration of vehicles from the `30s through the `80s. He is an author, magazine editor, car show judge and professional engineer. A member of several car clubs and a leader in some, through Anderson Automotive Enterprises (www.andersonautomotiveenterprises.com) he restores and appraises cars.
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