My bet is on the rebuilt master cylinder not being a good one, as everything else you checked looks ok. A rebuilt unit, if the brake pedal is pushed all the way to the floor while bleeding, can score the new seals and cause a leak due to the push rod having pits from previous water contamination. That is why it is suggested to put a brick, piece of wood, or your other foot beneath the brake pedal when bleeding brakes so that the push rod only sees natural travel. It takes longer, but doesn't score the seals...As MK378 said look for leakage down the booster beneath the master cylinder.

Good luck and please post your findings.

Ed Fisher
Dallas, Tx

 

Thanks for all the advice. I will go unbolt the MC.

 

It has a wet spot on the bottom of the seal. I really hate to have to get another MC again. The booster works just fine.

 

But for the amount of fluid I added (about 1cm in the MC) I would think there would be a lot more of a mess here. Maybe it's dripping down into the booster?

 

OK so I saw something dripping off a spring inside the master cylinder. Guess i'll replace the master cylinder. Here's my question. The booster works fine, I should just go ahead and use it till it quits? A bad booster won't cause a bad MC will it? The booster is really expensive.

 

No it won't cause a bad MC but when the booster blows out (when you least expect it) you will suddenly lose power brakes, and you will have to press VERY HARD on the brake pedal (possibly using both feet) to make the car stop.

 

FWIW I am going through the same loss of fluid with a new master cylinder (not a rebuild). I think that I have isolated it to the reservoir leaking. I will know more this weekend when I replace the replacement. Sigh. This is not a reman unit btw, but a new one.

 

Also, as I think you will see in your pictures above, the seal that fits around the shaft should prevent brake fluid from dumping into the booster. It is far more likely, I believe, to pour out around the mounting flange and down the front of the booster, which has been the case in every master cylinder leak that I have replaced. Brake fluid in the booster would show up as a smoke out of the exhaust I believe. However, I could be wrong about this, it is just a guess.

 

There is only one "o" in "losing".