Hope that someone has a clue on this one. My 89 has a strange brake problem. I sometimes lose the pressure in my bakes. The pedal wants to go to the floor. If I pump it once, the pressure is right back. I do not seem to be leaking any fluid. It may not happen for two weeks, and then happen 3 times in a drive, and then not happen for another 3 weeks. Any ideas?

 

Sounds like the Brake Booster Diaphragm has ruptured creating a vacuum leak.
Do you hear a whooshing sound when applying the brakes?
If you hear this hissing air sound, replace the Brake Booster.

or

The rubber hose between the brake booster and the engine is cracked or loose; creating a vacuum leak.

or

The Master Cylinder needs to be replaced.


With all of that said, I am wondering; why it does not do it all the time...


Maybe someone else will know.

See picture.

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If the pedal creeps down to the floor without losing your power brakes than your master cylinder is leaking internally. Half the time this will not leak past into the power booster and trash it. The seals on the master cylinder leak past into the bore of the cylinder, when you let off the brakes the fluid lost is pushed back into the reservoir like normal.

 

You might also want to look at the check valve on the booster