The main components of drum brakes are the brake drum, brake shoes, brake linings and wheel cylinders. The drum itself is the hollow metal cylinder situated at either end of the axle, immediately behind the wheel. The drum houses the brake shoes -- curved pieces of metal that push against the inner surface of the drum when the brakes are applied. The contact of the shoes to the drums is what causes your car to stop when you press on the brake pedal. On each brake, the two shoes are connected by an adjusting return-spring that pull them back away from the inner-drum surface when the brake is released. The shoes are covered by a brake lining, a durable heat-resistant covering that actually contacts the drum's inner surface. The wheel cylinder, part of the hydraulic system that causes the brake mechanisms to activate when the brakes are engaged, controls the process. Pistons in the wheel cylinder, activated by pressure in the brake-fluid system, press outward on the brake shoes so that they press against the drum, causing the wheels to stop. When drum brakes have become worn, the drums need to be removed to facilitate inspection of the inner surface. If the surface has become glazed or damaged, braking is compromised. If this is the case, the drums need to be either resurfaced (a.k.a., "turned") or replaced. Resurfacing is something that should generally be done by a brake technician since it requires a brake lathe -- a highly specialized piece of equipment. Brake shoes should be replaced every 30,000 miles or so as part of routine maintenance. The first step is to remove the drum itself. Since there is likely to be an abundance of loose brake dust behind the drum, it's a good idea to wear a particle mask while performing this part of the process (figure A). With the wheel lugs removed, carefully lift the drum free of the lug-bolts. Wiggling the drum gently from side to side on the mounting hub may help to loosen it from the bolts. With the drum removed, inspect the interior surface (figure B) for signs of abrasion or glazing. While the drum is off, the inside of the drum is cleaned with brake-cleaner spray. Brake cleaner is also used on the parts housed inside the drum, including the shoes, linings, and surrounding area (figure C). Before a drum can be resurfaced, it must be measured to determine whether there is sufficient thickness left so that removing a miniscule layer of the inner surface won't result in brake drums that are too thin. This measurement is taken with a drum micrometer (figure D), a device that measures the interior diameter of the drum in increments of thousandths of an inch. The measurements are visible on the micrometer gauge ( figure E). The specifications showing the minimum required measurements for a particular drum are stamped into the metal drum (figure F). If the drums require resurfacing, the process is done using a brake lathe. The lathe has hardened carbide blades that shave off tiny amounts of metal from both surfaces of the rotor as it turns. Precision controls allow the operator of the lathe to dial in and determine, in thousandths of an inch, exactly how much metal is to be removed. Since the brake lathe is such a specialized and elaborate piece of equipment, resurfacing of drums must generally be done by professionals at a brake shop. The second major part of the rear brake job is replacing the brake shoes. The movement of the shoes is controlled by a return spring (figure G). When the brakes are engaged, the shoes are pushed outward against the inside wall of the drum. When the brakes are disengaged, the return spring brings the shoes back to their resting position, away from the drum surface. In order to change the shoes, the return spring must be removed. In addition to the return spring, there are retaining springs and clips at the front and rear of the brake that hold the shoes secure. These must be released, using a specialized tool, (figure H) before the shoes can be removed. Once the springs and brake shoes have been removed, the parking-brake assembly is loosened and lowered out of the way (figure I). The wheel cylinder should be checked for wear, damage or signs of leakage. In our demonstration, the rubber dust boot on the old wheel cylinder (figure J) was worn and ruptured. The best remedy in that situation is simply to replace the old cylinder with a new one (figure K).To replace the cylinder, loosen the retaining bolt from behind the backing plate of the brake assembly. The newly resurfaced drum (figure L) will provide improved stopping ability once it's reinstalled on the brake system. Before reinstalling the drum, however, secure the new shoes in place (figure M), using the appropriate fasteners or mounting hardware. Once the shoes are in place and the retaining and return springs have been put back on (figure N), the resurfaced drum can be mounted and the wheel reinstalled on the vehicle. Important: The braking system is perhaps the most important system on your vehicle from a safety standpoint. Since improper repair or installation of critical elements in the braking system could ultimately result in a serious accident, it's vital to make sure that any brake job is done right. For this reason, many people leave brake work to professionals at the auto dealership or a brake specialty shop. In addition, there is considerable variation among braking systems on different vehicles, particularly on newer vehicles that use sophisticated hydraulic and electronic control systems. Simple elements in brake repair, such as replacing pads or shoes, are jobs that the automotive do-it-yourselfer can do -- provided that the instructions in the vehicle's service manual are consulted and followed. Major brake work, however, generally is best left to professionals who have the expertise and equipment to do it properly. The description of the steps in this project is intended to provide an understanding of some of the procedures that are involved in typical brake repair.
RESOURCES :
Auto Repair for Dummies
Author: Deanna Sclar
ISBN: 0764550896
Vehicles provided by McNelly-Whaley Ford Lincoln Mercury
750 Dolly Parton Parkway
Sevierville, TN 37862
Phone: 865-453-2833
Web site: www.ford4u.com
Automotive Brakes and Antilock Braking Systems
Model: 0314028382
Author: Kalton C. Lahue
(1995)
To order this title from Amazon.com, click here.
Delmar Learning
Brake Handbook
Model: 0895862328
Author: Fred Puhn
(1985)
Order this book from Amazon.com.
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