Master Cylinder

Usually, the first and strongest indicator of anything wrong in the brake system is a feeling through the brake pedal. In diagnosing the condition of the master cylinder, the Master Cylinder Diagnosis charts, not only make use of pedal feel as evidence of a brake problem, but it also employs two other problem indicators, such as the illumination of the brake warning lamp and the brake fluid level in the master cylinder's reservoir.

Note that the procedures refer to the use of diagnostic techniques. These techniques (Numbers 1 through 4) follow the diagnostic procedures.

Normal Conditions

The following conditions are considered normal and are not indications that the master cylinder is in need of service.

Condition 1: New brake systems are not designed to produce as hard a pedal effort as in the past. Complaints of light pedal efforts should be compared to pedal efforts on another vehicle, same model and year.

Condition 2: During normal operation of the master cylinder, the fluid level in the reservoir will rise during brake application and fall during release. The net fluid level (i.e., after brake application and release) will remain unchanged.

Condition 3: A trace of brake fluid existing on the booster shell below the master cylinder mounting flange. This results from the normal lubricating action of the master cylinder bore end seal.

Condition 4: Fluid level will decrease with pad wear.

Abnormal Conditions

Changes in brake pedal feel or travel are indicators that something could be wrong in the brake system. The diagnostic procedures and techniques use brake pedal feel, the warning indicator illumination, and brake fluid level as indicators in diagnosing brake system concerns.

Condition 1: Pedal goes down fast. This could be caused by an external or internal leak.

Condition 2: Pedal eases down slowly. This could be caused by an external or internal leak.

Condition 3: Pedal is low and/or feels spongy. This condition may be caused by no fluid in the reservoir, reservoir cap vent holes clogged, rear brakes out of adjustment, or air in the hydraulic system.

Condition 4: Pedal effort excessive. This may be caused by a bind or obstruction in pedal/linkage, clogged fluid control valve or insufficient booster vacuum.

Condition 5: Rear brake lockup during light pedal force. This may be caused by wrong tire pressure, grease or fluid on linings/damaged linings, improperly adjusted parking brakes, or damaged/contaminated pressure control valve(s).

Condition 6: Erratic pedal effort. This condition could be caused by brake booster malfunction, extreme caliper piston knock back or improperly installed disc brake shoe and lining.

Condition 7: Brake warning indicator ON. This may be caused by low fluid level, ignition wire routing too close to fluid level indicator assembly, or float assembly damage.

NOTE: Prior to performing any diagnosis, ensure that the brake system warning indicator is functional.

The diagnostic techniques and service procedures are referenced in the Brake Master Cylinder Diagnosis charts. Refer to these charts for proper use of diagnostic techniques in diagnosing brake hydraulic system concerns.