Check the brake drum for cracks, heat cracks. heat checks, hard spots, scoring, or worn beyond specifications. Damaged drums may be machined (turned) as long as they still meet the manufacturer's specifications. Badly damaged or worn drums must be replaced.
To check the wheel cylinder for leakage, pull back the cylinder boots. If the boot is full of fluid, the wheel cylinder should be rebuilt or replaced. Also, check the return springs and the automatic adjusting mechanism.
When major brake service is being performed, the master cylinder is to be inspected for proper operation. A faulty master cylinder usually leaks externally out the rear piston or leaks internally. You are able to detect external brake fluid leaks by checking the master cylinder boot for fluid or dampness on the firewall. When the leak is internal. the brake pedal will slowly move to the floor. Inoperative valves in the master cylinder are also a reason for service.
To remove the master cylinder, disconnect the brakes lines from the master cylinder using tubing wrenches. With the brake lines disconnected, unbolt the master cylinder from the brake booster or firewall. In some cases. the pushrod must be disconnected from the brake pedal.
Many shops, however, simply, replace a bad master cylinder with a factory rebuild or a new one. A replacement master cylinder is normally cheaper than the labor cost and parts for an in-shop rebuild.
NCF units require replacement of faulty master cylinders. Rebuilding of master cylinders is NOT authorized.
To rebuild a master cylinder, drain the fluid from the reservoir. Disassemble the master cylinder following the instructions in the manufacturer's service manual. After disassembly, clean the parts in brake fluid or a recommended cleaner.
Do NOT clean the hydraulic parts of the brake system with conventional parts cleaners. They can destroy the rubber cups in the brake system. Only use brake fluid or a manufacturer's suggested cleaner (denatured alcohol. for example).
If the cylinder is not pitted, scored, or corroded badly, it may be honed using a cylinder hone. When the cylinder is honed, the hone is ran ONLY once in and out. After honing, measure the piston-to-cylinder clearance, using a telescoping gauge and an outside micrometer or a narrow (1/8" to 1/4" wide) 0.006" feeler gauge. When a feeler gauge is used, if the gauge can be inserted between the cylinder wall and the piston, the master cylinder must be replaced. The cylinder must NOT be tapered or worn beyond the manufacturer's specifications. Replace the master cylinder if the cylinder is not in perfect condition after honing.
Blow-dry all parts with low-pressure compressed air. Blow out the ports and check for obstructions. Lubricate all parts with the recommended brake fluid and assemble the master cylinder, using the manufacturer's service manual.
After the master cylinder is reassembled, it is good practice to bench bleed a new or rebuilt master cylinder before installation on the vehicle. A master cylinder is bled to remove air from the inside of the cylinder. Bench bleeding procedures are as follows:
Mount the master cylinder in a vise
Install short sections of brake line and bend them back into each reservoir
Fill the reservoir with approved brake fluid
Pump the piston in and out by hand until air bubbles no longer form in the fluid
Remove the brake lines and install the reservoir cover
Once the master cylinder has been bench bled. it is ready to be reinstalled on the vehicle. Bolt the master cylinder to the booster or firewall. Check the adjustment of the pushrod if there is a means of adjustment provided. Without cross threading the fittings, screw the brake lines into the master cylinder, and lightly snug the fittings. Then bleed (remove air from) the system. Tighten the brake line fittings. Refill the reservoir to the proper level and check brake pedal fall. Last but not least, test the vehicle.
You should understand the most important methods for servicing a drum brake. However. specific procedures vary and you should always consult the manufacturer's service manual. Brake service is required anytime you find faulty brake components. A leaking wheel cylinder, worn linings, scored drum, orContinue Reading