We have a 1990 Bronco 5.8 4x4. We replaced the master cylinder and when we tried to bleed the brakes, it started leaking where it bolts to the brake booster. After 2 rebuilt and 2 new, it still leaks. We have replaced the proportioning valve and readjusted the pushrod on the brake booster, It Still Leaks. No one can tell us what the problem is

 

Ok, outlandish as this may sound...was the original master cylinder replaced for the same reason? If so, was the booster checked for brake fluid contamination? Not 100% certain on the plausability of this but is it possible that what you are seeing the the booster trying to purge fluid that the original master cylinder leaked into it?

Guys, help me out on this one...is this even worth checking? I would think that if the original MC was replaced for this reason, it may have been doing it for a while before it was replaced and it bled brake fluid into the booster.

 

We replaced the master cylinder because when at a light, the brake pedal would go to the floor. After bleeding it still hapened so we replaced the master and that started a 2 day,4 cylinder marathon. Every one says its the master cylinder but 4 can't be bad.

 

The original cylinder did not leak, we replaced it because it would'nt hold pressure.

 

Well that is a tell-tale symptom of a dead/dying master cylinder. However if the booster HAS taken on fluid you may see intermittent working from it because fluid inside the booster will slowly deteriorate the diaphram in the booster. The diaphram seal is designed to handle plain ol' air and will eventually disntegrate if exposed to brake fluid. Are you certan there are no leaks anywhere else? Rear wheel cylinders? calipher pistons? partially open bleed valve. ABS equipped vehichles will have at least one bleed valve on that part of the system too. Rear wheel cylinders will only make failures known by spewing fluid all over the inside of the drum and eventually all over the wall of the tire facing the inner fender.

Original "wouldn't hold pressure"? And you are certain that it didn't leak back inside the booster? Hydraulics are pretty cut and dried either they hold or they leak. If it won't hold pressure there either are no seals or the seals won't contain the fluid anymore. Either way the fluid is getting past the seals somewhere and going somewhere it shouldn't.

 

The original did not leak, we have no fluid at the wheels and none (until now) on the garage floor. When we replaced the first one their was no fluid on the brake booster. We are bench bleeding the 5th one now. All the times before it did'nt start leaking until after the back brakes were bled. and we started on the front.

 

I ask my husband why we replaced the master cylinder. He said the brakes were mushy and he called our local parts store and they said the master cylinder was bad and it needed to be replaced.

 

I had to replace the master cylinder in my '85 for exactly the same reason. When I was sitting at a stop light the pedal would slowly press down to the floor. If I didn't let my foot off the pedal and re-apply the brakes I would start slowly creeping forward. I was fortunate in that I didn't have any problems with the new master cylinder. Then again, I didn't do it myself, I had a friend who owns a brake shop do the work.

 

You most likely have a bad booster that is leaking vacuum to the seals of the master cylinder. It will suck the fluid out of the master cylinder and when you cut the motor off, it leaks to the outside. My 91 Bronco did this to me. I went thru 3 master cylinders before I found the problem.