Brake Master Cylinder Troubleshooting

Troubleshooting a bad master cylinder requires a visual inspection and a pressure test. During the visual inspection, external leaks can be found and clogged ports can be identified. During the pressure test, internal parts can be checked for wear and damage. Both sets of checks must be done to insure accuracy in diagnosis.

System Operation

A typical master cylinder is made up of two pistons, located inside a bore in the housing, and a fluid reservoir. Each piston has primary cup seal and a secondary cup seal, and a return spring. When at rest in the bore, the primary cup seal sits between the vent and replenish ports, connecting the bore and fluid reservoir. As the piston is pushed forward in the bore, the primary seal moves past the vent and replenish ports to close the hydraulic system. Fluid is allowed to fill in the space behind the pistons through the replenish ports, and when the pedal is released, fluid moves back into the master cylinder through the replenish ports.
A clogged vent or replenish port can cause a master cylinder failure.

Clogged Ports and Leaks

The visual inspection begins with a look at the master cylinder. Any wetness near the rear of the master cylinder at the booster needs a closer look. Remove the master cylinder retaining nuts, and check for fluid between the cylinder and the booster. Any fluid here indicates a leaking master cylinder, and a replacement will be needed.
If no external leaks are found, clean and remove the reservoir cap. Check the fluid level and condition. Then have a partner push the brake pedal as you observe the fluid. When the pedal is released, a spurt of fluid should be observed in the reservoir. No spurt of fluid indicates a clogged replenish port, and the only repair is a new cylinder. Unless the fluid can release into the master cylinder reservoir when the pedal is released, the pressure cannot be relieved and will hold the brakes on only partially. Another check for this is to apply the brake, released it and open the bleeder screw. A clogged vent or replenish port will cause fluid to spurt out of the bleeder.

Pedal Pressure Testing

Testing for pressure bypass begins with the engine in the off position. Apply the pedal several times to eliminate residual power brake booster assist. Apply hard pressure to the pedal and hold. The pedal should hold and not drop. If the pedal drops in this position, there is a leak somewhere else in the system.
Without releasing the pedal completely, release the pressure until light pedal pressure is applied. The pressure should be about the same as what you would apply when holding at a stop light. If the pedal drops now, the master cylinder is bypassing pressure internally and should be replaced.