Rick's Free Auto Repair Advice

Test master cylinder

How to test a master cylinder

When you apply the brakes and the pedal goes to the floor, most mechanics and DIYers automatically assume they’re dealing with a bad master cylinder (MC). But unless you actually test it, you’re just guessing, because some ABS components can mimic the feel of a bad MC. A stuck open ABS valve to an ABS accumulator can allow brake fluid to flow out of the MC and fill the accumulator. That would feel exactly like a bad MC.

Test master cylinder

So the only way to confirm a bad MC is to remove all the brake lines and plug the ports. Some mechanics make plugs with dummy brake lines that have been soldered shut. But you can make your own plugs simply by cutting small circles out of rubber and inserting them in the port. Then snug up the brake line. Don’t overtighten or you’ll cut the rubber and force it down the brake line. All you’re trying to do is block fluid flow. And yes, the brake fluid will make the rubber swell. But they’ll only be in the ports for a few minutes for the test. Then you can toss them. If the pedal still goes to the floor with the ports plugged, you’ve got a bad MC—one where the fluid is bypassing the cup seals in the bore. But if the pedal stays firm, your problem is elsewhere.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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