Complete drum brake service typically involves:
1. Removing parts from backing plate.
2. Cleaning and inspecting parts.
3. Replacing brake shoes.
4. Replacing or re-building (Repairing) wheel cylinders).
5. Turning (resurfacing) brake drums.
6. Lubricating and reassembling brake parts.
7. Pre-adjusting. Bleeding and testing brakes. Drum Brake Disassembly To disassemble drum brakes, re-move the tire and wheel assemblies and brake drum. If the drum is rusted to the axle flange light taps with a hammer may be needed. Hammer on the outside edge of the drum. Do not hammer on the inner lip of the drum otherwise it will brake.
Use a brake spring tool to remove the upper shoe return springs organize the springs so that they can be installed in the same location. The primary and secondary springs are usually of a different colour and tension. Use a hold down spring tool to remove the hold down springs.
Lift the brake shoes off of the backing plate. Remove the Automatic adjuster mechanism. Before disassembling the wheel cylinder clean the backing plate. Wipe off the backing plate with a cotton rag. If coated with brake fluid or axle lubricate, wash the backing plate with an approved cleaner. Servicing Wheel Cylinders Pull back the wheel cylinder boot. The wheel cylinder should be repaired or replaced if it shows any signs of leakage or sticking. In fact, every time the linings are replaced wheel cylinders are serviced.
To repair (rebuild) a wheel cylinder, remove the boots, pistons, cups and springs; usually the wheel cylinder can be serviced while it is bolted to the backing plate. A wheel cylinder repair normally involves honing the cylinder and replacing the rubber cups and boots.
It is very important that the inside of the cylinder should be in good condition. Any sign of scratches scoring or pitting requires cylinder replacement. To hone a wheel cylinders mount the small hone in an electric drill. Insert the hone in the cylinder. Turn on the drill while moving the hone back and forth in the cylinder. Keep hone lubricated with brake fluid.
After honing, clean the wheel cylinder thoroughly with clean cotton rags and brake fluid or an approved cleaning solvent. Make sure the cylinder is clean and in perfect condition before reassembly. The slightest lit of grit or roughness will cause cup leakage.
Make sure the new wheel cylinder cups are of the some size as the old ones. Cup size is normally printed on the face of the cup. Brake Drum Resurfacing Brake drum resurfacing is also called drum turning. It involves the machining of the friction surface of the drum on a brake lathe. Resurfacing is needed when the drum is scored, out-of-round or worn unevenly. The drums are machined (turned) almost every time when the brake linings are replaced.
To re-surface a brake drum, mount the drum on the lathe. Follow the operating instructions for the particular type of lathe. Wrap a silencing band, which is a rubber or spring strap, around the outside of the drum. It will prevent vibration, which will affect drum surface smoothness. Wear on eye protection.
Feed the cutting tool against be inner surface of the drum. Adjust the depth of the cut to lathe specifications and activate the automatic feed. When surfacing a drum, machine as little material off as possible. Re surfacing thins the metal around the friction surface. As a result, the drum is more prone to overheat and warp.
Machine the right and left hand side drums to same diameter. Due to which the braking of the vehicle will be straight line. Brake Drum Grinding Sometimes brake drum grinding is needed which is done by using grinding stone instead of cutter. It is used to remove hardened area in the drum. The grinder is mounted on the lathe instead of cutter arm. Measuring Brake Drum Diameter Typically, a brake drum should not be more than 0.06” (1.5mm) over size. For example a drum that is 9 inch (229 mm) in diameter, when new, it must not be over 9.060 inch (230mm) after re-surfacing. If it is large in diameter, the drum is dangerous to use.
To measure brake drum diameter, use a special brake drum micrometer. It will measure drum diameter quickly and accurately. Replace the drum if it is worm more than specifications. Sometimes maximum diameter is stamped on the side of the brake drum. Drum Brake Reassembly To re-assemble drum brakes lubricate the small pads or lumps on the backing plate. This will keep the shoes from squeaking. Avoid using too much lubricate other wire the lining will be contaminated and revised.
Before installing the new shoes, check their fit inside the brake drum. There should be a small clearance between the ends of the lining and drums. The shoes should rock slightly when moved in the drum. If the center of the linings is not touching the drum the linings should be arced (ground).
When arcing brake shoe linings, full the instructions provided by the equipment and vehicle manufacture. Basically, linings are grinded 0.35 inch or 0.89 mm smaller in diameter than the drum. Without clearance, the lining will chatter and vibrate when returned to service. Caution Make sure the vacuum system on the lining grinder is working properly. Reminder asbestos brake lining dust can cause cancer. Install the new shoes and the adjust mechanism on the backing plate. Make sure all of the parts are positioned correctly, check the following:
1. Are the wheel cylinders in perfect condition and assembled properly.
2. Has the backing plate been lubricated?
3. Is the primary (smaller) lining facing the front of the car and the secondary (larger) facing the rear? Are the shoes central on the backing plate and contacting the anchor correctly?
5. Are all springs installed properly?
6. Does the automatic adjuster work.
7. Are the lining surfaces perfectly clean (sand if needed)?
8. Do the brake need bleeding?
Similar illustrations are contained in service manuals. Use them to help position the parts on the backing plate correctly. Pre-adjusting Drum Brakes To pre-adjust the brake shoes, fit a brake-adjusting gauge in to the brake drum. Set the gauge for the inside diameter of the drum. Tighten the lock on the gauge. Fit the gauge over the brake shoe. Then turn the star wheel or move the adjuster arm until the lining touches the gauge. This will pre-adjust the linings the correct distance from the inside of the drum. Another way to pre-adjust drum brakes involves use of a brake spoon (star wheel tool) to turn the adjuster or star wheel. Turn the star wheel until the brake drums drag lightly when turned by hand. Parking Brakes Adjustment To adjust the parking brake an adjustment nut is normally tightened on the cable mechanism. During adjustment, release the parking brake lever or pedal, lubricate cables and linkages. To prevent over adjustment, engage the parking brake one notch. Then, turn the cable adjuster to remove excess slack. Operate the emergency brake and make sure the brakes are not dragging when released.