soft brake pedal
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soft brake pedal
Recently , my brake pedal started sinking slowly at traffic lights. I'm guessing that this is due to a loss in vacuum pressure. Before I replace the master cylinder or booster, I was looking for a way to test and trouble shoot the exact cause of this problem. Any recommendations would be greatly appreciated.
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
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From: Simi Valley, CA
Recently, my brake pedal started sinking slowly at traffic lights. I'm guessing that this is due to a loss in vacuum pressure. 
Before I replace the master cylinder or booster, I was looking for a way to test and trouble shoot the exact cause of this problem. Any recommendations would be greatly appreciated.
Before I replace the master cylinder or booster, I was looking for a way to test and trouble shoot the exact cause of this problem. Any recommendations would be greatly appreciated.
Master cylinder leaks can be hard to detect, since they usually leak into the brake booster.
Posting Guru
Joined: Apr 2006
Posts: 1,477
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From: NAMPA, ID.
I'd say it is very safe to assume that the master cylinder is internally bypassing which is not uncommon. A wheel cylinder or any other external leak will obviously result in a puddle somewhere or fluid contamination of the shoes and/or drums. A bad master cylinder will not result in an external leak so unless your fluid level is dropping and you don't see fluid anywhere, you've got a bad master cylinder. You can rebuild it but an auto parts store rebuilt unit is ridiculously inexpensive.
Lead Driver
Joined: Apr 2011
Posts: 6,399
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From: El Dorado, Arkansas
I would not fool with a rebuilt MC, unless you just like changing MCs out often. They have a poor track record for lasting. New ones are not expensive and generally have a much longer service life.
If you pull the MC loose from the booster and see that brake fluid has leaked out the back end and over into the booster, the booster will need to be replaced too. Brake fluid and the rubber diaphragm of a booster don't get along. The fluid will deteriorate the diaphragm and it's just a matter of time until the booster will fail.
If you pull the MC loose from the booster and see that brake fluid has leaked out the back end and over into the booster, the booster will need to be replaced too. Brake fluid and the rubber diaphragm of a booster don't get along. The fluid will deteriorate the diaphragm and it's just a matter of time until the booster will fail.
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