Brake Master Cylinder: Testing and Inspection

Inspect master cylinder bore for scoring or corrosion. It is best to replace a corroded cylinder. Corrosion can be identified as pits or excessive roughness.

NOTE: Polishing the bore of the master cylinder with the cast aluminum body with anything abrasive is prohibited, as damage to the cylinder bore may occur. Rinse cylinder in clean brake fluid. Shake excess fluid from cylinder. Do not use a cloth to dry cylinder, as lint from cloth cannot be kept from cylinder bore surfaces. Make certain that compensating port in cylinder is clear. If deep scratches or heavy corrosion is noted, the cylinder should be replaced.

These tests will not determine all master cylinder malfunctions. Use the Brake Diagnosis Table and Chart to help isolate the problem if it is not resolved by these tests.
1. Check for a cracked master cylinder casting or brake fluid around the master cylinder. Leaks are indicated only if there is at least a drop of fluid. A damp condition is not abnormal.
2. Remove fluid reservoir cap and diaphragm and depress service brake pedal. A "geyser" should be seen in the reservoir as the brake pedal is depressed. If the system fails this test, check for binding pedal linkage and incorrect push rod length. If both of these are 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 is suspected. If contaminated, all components should be disassembled and cleaned. All rubber components should be replaced and all pipes flushed.