Brake Master Cylinder: Testing and Inspection

These checks will not determine all master cylinder malfunctions. Use the Brake System Diagnosis By Symptom chart to help isolate the problem if it is not found by using these tests. Testing and Inspection

With the engine running at idle and the shift lever in neutral, depress the brake pedal and hold a constant foot pressure on the pedal. If the pedal gradually falls away with the constant pressure, the hydraulic system may be leaking. Perform a visual inspection to confirm any suspected leak. Check the master cylinder fluid level. While a slight drop in reservoir level does result from normal lining wear, an abnormally low level in either reservoir may indicate a leak in the system. The hydraulic system may be leaking either internally or externally. Also, the system may appear to pass this test but still have slight leakage. If the fluid level is normal and no external leaks are found at the brake pipe or hose connections or at the caliper or wheel cylinder boots, check the service brake pedal travel, brake adjustment and the parking brake adjustment.

1. Check for a cracked master cylinder casting or brake fluid around the master cylinder. Leaks are indicated even if there is only a drop of fluid. A damp condition is not abnormal.

2. Check for a binding pedal linkage. If satisfactory, disassemble the master cylinder and check for swollen or elongated primary piston seal(s). If swollen seals are found, substandard or contaminated brake fluid should be suspected. If contaminated, all components should be disassembled and cleaned; all rubber components should be replaced and all pipes flushed.

NOTE: Use only Delco Supreme II Brake Fluid GM P/N 1052535, or equivalent DOT-3 brake fluid. Do not use power steering or transmission fluid in the brake system. Do not re-use brake fluid accumulated during brake system bleeding. Always store brake fluid in a closed container.
Re-seal brake fluid containers immediately after use. Do not use brake fluid left in an open or improperly sealed container because it absorbs moisture or can become contaminated.