A brake booster’s malfunction may come from a small leak

April 1, 2016 | By Staff


In John Veatch’s inquiry about a hard pedal in the January issue, he states that the booster is “functional” since he gets a pedal drop when the engine is started. I recently replaced the booster in my ’76 Jaguar XJ6C and had the same problem…hard pedal. I did the same basic test and found that I had pedal drop once the engine started.

I went on to replace pads and one brake line that was suspect but there was no difference. I read somewhere that “glazed” pads might cause a hard pedal. I also read that a bad check valve could cause problems so I tested the check valve which is inline on my Jag by removing it, cleaning it, then blowing through it easily in one direction and then not being able to in the other. Put the check valve back in and, of course, no change, still a hard pedal.

There was no reason to think that testing the check valve would fix anything, but I’m always hopeful that examining something may change something. I put a tee between the check valve and the booster and attached a vacuum gauge. I got 19” of vacuum, which is good, turned the car off, and the vacuum held for several minutes. This says that overall the booster is sealed.

I pressed the brake pedal and watched the gauge slowly drop. This says that there is a leak between the vacuum side and the ambient side of the booster. To review, a booster has two chambers or “sides,” vacuum and ambient, separated by a diaphragm. The vacuum side is sealed and evacuated by engine vacuum. The ambient side is sealed whenever the brake is NOT applied. When the brake is applied, the ambient side seal is broken and the ambient side of the diaphragm is exposed to ambient atmospheric pressure.

This pressure difference pushes the diaphragm toward the vacuum side, which pushes the brake pushrod into the master cylinder, providing boost. This may be an oversimplification but you get the gist. Anyway… It turns out that a small hole or tear in the diaphragm allows leakage from the ambient to the vacuum sides of the diaphragm while still supplying a portion of the required boost.

I returned the booster and replaced it and my brakes work fine. So, I would not rule out the booster because it passes a basic test. A brake booster not only needs sustained vacuum, but it needs to sustain a “large volume” (if you can call it that) of vacuum. During a basic test on a leaking booster, there is enough vacuum available in the booster to provide initial pedal drop, but after that there’s no more boost available. Depending on the size of the leak, you may even get adequate boost to begin braking then lose it as the vacuum side fills with ambient pressure commonly called “fade.”

John may very well have a proportioning valve problem, but before breaking the seal on hydraulic lines and getting into all of that, I would attach a vacuum gauge first and check for booster “leak down.” It’s simple and takes little time if you have an in-line check valve. If not, it takes a little more effort to add one, but it will be well worth it.


Thanks for this clearly written information, Steve. Good input from readers is helpful to all of us. We are all in this hobby together, and if we can help each other with valuable problem-solving information, it makes Auto Restorer even more valuable.