Booster Or Master Cylinder?

Nail-er · April 25, 2008

2000 GMC Sierra 5.3 liter vacuum booster

I had to make a panic stop yesterday ( dry roads) I was going 35 MPH . The ABS kicked in then the brake pedal dropped to an inch from the floor. I nearly rear ended some one. The pedal stays an inch from the floor. Pumping does not bring it up. Braking hard brings me to a gradual stop.

Booster and master cylinder are $100 each. Any thoughts as to which one is shot?

Will booster falure cause pedal drop?

I hate ABS brakes. I think they cause more accidents than they save.

Is there a way to bypass ABS without causing fault codes? Can I put a switch in to turn it off on dry streets?

Zembonez · April 25, 2008

It's hard to say. Here are some random thoughts.

Is the pedal firm at the "one inch from the floor" level? If so, it might be a boost issue. (Usually, a master cylinder will "pump up" if you pump the brakes even if it is bypassing internally) A bad master cylinder will usually slowly go to the floor.

Did the problem happen all at once? If so, maybe the diaphram in the booster ruptured?

Have you checked all of your lines for leakage? Usually air in the lines will allow you to pump up the system too though...

Try this: Run the engine for a few minutes to build vacuum. Shut it off and immediately pull off the vacuum line to the booster. If you hear the "whoosh" of escaping vacuum, the diaphram is likely ok. If it is torn, the vacuum will escape and when you pull the hose, there will not be any vacuum present. Not entirely scientific but boosters are pretty simple devices.

Good luck man. Get it checked out before you becoma a statistic!

Nail-er · April 25, 2008

It's hard to say. Here are some random thoughts.

Is the pedal firm at the "one inch from the floor" level? If so, it might be a boost issue. (Usually, a master cylinder will "pump up" if you pump the brakes even if it is bypassing internally) A bad master cylinder will usually slowly go to the floor.

Did the problem happen all at once? If so, maybe the diaphram in the booster ruptured?

Have you checked all of your lines for leakage? Usually air in the lines will allow you to pump up the system too though...

Try this: Run the engine for a few minutes to build vacuum. Shut it off and immediately pull off the vacuum line to the booster. If you hear the "whoosh" of escaping vacuum, the diaphram is likely ok. If it is torn, the vacuum will escape and when you pull the hose, there will not be any vacuum present. Not entirely scientific but boosters are pretty simple devices.

Good luck man. Get it checked out before you becoma a statistic!

Thanks,

I figured there must be a way to test the booster. I'll give it a try. It was scarey having the brakes drop out just when I needed them. Good thing I wasn't on the hiway.

Thanks again