Rick's Free Auto Repair Advice

Diagnose a brake master cylinder problem

How to accurately diagnose a brake master cylinder problem

If your brake pedal sinks to the floor during braking you have to first determine whether the problem is in the brake master cylinder or somewhere else in the braking system. Start by checking for brake fluid leaks at each wheel, distribution block, and all the brake lines. If you can’t find any leaks and you’re not losing any brake fluid between checks, chances are you have an internal brake master cylinder seal failure. To confirm that, you’ll have to diagnose the brake master cylinder.

To diagnose a brake master cylinder you’ll need to plug the lines

The easiest way to plug the brake master. brake master cylinder

brake master cylinder plugs

Brake master cylinder plugs

cylinder links is to purchase a package of brake master cylinder plugs. The plugs install in place of the individual brake lines and isolate the brake master cylinder from the rest of the braking system.

If you’re diagnosing a brake master cylinder problem on an existing master cylinder, simply remove each brake line from the master cylinder and install the plugs. Then apply firm pressure to the brake pedal. The brake pedal should sink a bit and then stop. If it continues sinking to the floor, the master cylinder must be replaced due to internal seal failure.

If you’ve replaced the master cylinder and the pedal is sinking to the floor, you must first perform a bench bleed procedure to remove all air from the master cylinder. See this video for the procedure.

After performing the bench bleed, reinstall the master cylinder on the vehicle and install the plugs. If the pedal still sinks to the floor, the master cylinder is defective.

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat

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