Brake Master Cylinder Tests

Brake Master Cylinder Tests

Bypass Condition
1. Check the brake hydraulic system for leaks or insufficient brake fluid. Repair as necessary.
2. Observe the brake fluid level in the brake master cylinder reservoir while the brake pedal is slowly applied and released. If the brake fluid level drops when the brake pedal is applied and rises when the brake pedal is released, but the net brake fluid level remains unchanged, the brake master cylinder is bypassing. Repair or install a new brake master cylinder.

Non-Pressure External Leaks
A low brake fluid level in the brake master cylinder reservoir may be caused by these non-pressure external leaks:

Type 1: An external leak may exist at the brake master cylinder filler cap due to incorrect position of the cap, cap gasket or cap gasket deterioration. Repair or install new components as necessary.

Type 2: An external leak may exist at the brake master cylinder mounting flange due to internal seal failure. Repair or install a new brake master cylinder.

Type 3: An external leak may exist at the base of the brake master cylinder reservoir due to deteriorated reservoir mounting seals. Repair or install a new brake master cylinder.

Compensator Port
The purpose of the compensator ports in the brake master cylinder is to:
^ supply additional brake fluid from the brake master cylinder reservoir needed by the brake system due to brake lining wear.
^ allow brake fluid to return to the brake master cylinder reservoir when the brakes are released. The returning brake fluid creates a slight turbulence in the brake master cylinder reservoir. This is a normal condition and indicates that the compensator ports are not clogged.

Clogged compensator ports may cause the brakes to hang up or not fully release. If clogged compensator ports are suspected, proceed as follows:
1. With the vehicle in NEUTRAL, position it on a hoist. Refer to Maintenance/Service and Repair.
2. With the brakes released, attempt to rotate each wheel. If an excessive amount of brake drag exists, continue with the test. If an excessive amount of brake drag exists at only one wheel, it indicates a possible seized brake disc caliper or parking brake component. Repair or install new components as necessary.
3. Check the brake stoplamp switch adjustment and the brake pedal free play to verify that the brake pedal is not partially applied.
4. Loosen the nuts attaching the brake master cylinder to the power brake booster and pull the brake master cylinder away from the power brake booster.
5. Repeat Steps 1 and 2. If the brake drag disappears, the power brake booster is binding and a new one must be installed. If the brake drag continues, the brake master cylinder is binding and a new one must be installed.