MC Leak, Fluid Coming out of Brake Booster

porwolf · 08-12-2013, 07:17 PM

I discovered a sizable brake fluid leak in my 79 SC. First indication was that I noticed quite a bit of brake fluid being consumed without seeing any obvious leaks under the car or near the inside of the wheels. Then I found that my driver's side front carpet was soaked in something other than water. On closer inspection I found some faint brake fluid wetness under the brake booster - master cylinder connection. I removed the master cylinder + brake booster and separated them. And sure enough quite a lot of brake fluid in the front part of the brake booster where the master cylinder connects. But I also noticed a trail of corroded brake booster black paint under the part where the two elements connect.

Question: How does the brake fluid leak out of the brake booster where the master cylinder connects? That connection is supposed to be vacuum sealed. The O-ring there was in place and seemed to be OK. Performance wise my brake system showed no sign of degradation.

gregwils · gregwils's Garage

There is a diaphragm inside the booster that creates the seal, but I assume the booster housing could hold fluid. It's been a while since I had mine apart to remember exactly.

How old is your master cylinder?

Dr J · 08-13-2013, 03:46 AM

I had something similar to this happen to me. It destroyed my front driver's side mat and stained a bit of the carpet.

KTL · 08-13-2013, 05:45 AM

The master cyl has a pushrod sticking out of it that aligns with the pushrod in the diaphragm of the booster. Booster has a rod and clevis that connects to the alloy support in the trunk that connects to the brake pedal and that's how the master gets its input. So, there has to be a fluid seal on the master pushrod that has failed and that allowed the fluid to get into the booster.

New boosters from the dealer are listed as NLA by Pelican but show up on other Porsche dealer parts searches. Remanufactured ones are available from aftermarket sources like Centric and Cardone. I have a couple good used boosters from an '87 and '84 Carrera if you need one. The Carrera booster is larger than the SC by 1" but it'll still work OK. Current parts listing lists the Carrera booster as the replacement for the SC.

porwolf · 08-13-2013, 02:18 PM

I understand how brake fluid gets from the master cylinder into the brake booster via a defective master cylinder shaft seal. What puzzzles me is that the brake fluid can leak out of the vacuum sealed connection between the brake booster and the master cylinder. Logic tells me that if the fluid in the brake booster reaches the top level of the vacuum hose the brake fluid should only get sucked into the engine but not leak out at the connection betweee the master cylinder and the brake booster. In my case the rubber o-ring seal seemed to have been OK, the brakes worked fine, and I had no indication of a vacuum leak on my SC.

Any ideas?

KTL · 08-13-2013, 02:21 PM

Point well taken RE how leak into booster could find its way out. So I would venture to guess the master is leaking in more than one place?

porwolf · 08-13-2013, 02:37 PM

Quote:

Originally Posted by KTL

Point well taken RE how leak into booster could find its way out. So I would venture to guess the master is leaking in more than one place?

In my case, after taking the whole brake assembly apart on the bench I could clearly see the leak area on the front side of the master cylinder where the rod exits. I even saw metal grindings sticking to that surface and it was wet from brake fluid. So, that is where the brake fluid entered into the vacuum chamber of the brake booster. The question is how did the brake fluid exit the brake booster through the vacuum seal into the front trunk area?

porwolf · 08-13-2013, 02:40 PM

Quote:

Originally Posted by KTL

Point well taken RE how leak into booster could find its way out. So I would venture to guess the master is leaking in more than one place?

I also could clearly see the evidence of brake fluid running down on the outside of the brake booster right under the connection to the master cylinder. The black paint on the brake booster there was clearly corroded from brake fluid.

motogman · 08-13-2013, 03:15 PM

The rear seal on the master cylinder is meant to keep the fluid inside the master cylinder but if this seal wear or get a small cut in it, the vacuum in the booster will suck the fluid into the booster.

Usually the brake light will come on and the reservoir is found to be low. Investigation showing no other obvious leaks then leads to this internal leak.

The booster itself is a vacuum vessel that is evacuated continuously from the big pump in the rear of the car. It is not completely sealed or leak proof and as is well understood, brake fluid is not nice to paint and metal so the normal small leak paths, the action of the brake fluid and the physical forces for vacuum and pedal apply open up small leak paths. This normally not an issue to the function of the booster because the air pump is pretty substantial (3.0 L in volume for the SC).

Not to others... when your brake fluid goes away and you can't figure out where it went - look in the booster before it leaks on your Gucci's.

porwolf · 08-13-2013, 03:50 PM

Quote:

Originally Posted by motogman

The rear seal on the master cylinder is meant to keep the fluid inside the master cylinder but if this seal wear or get a small cut in it, the vacuum in the booster will suck the fluid into the booster.

Usually the brake light will come on and the reservoir is found to be low. Investigation showing no other obvious leaks then leads to this internal leak.

The booster itself is a vacuum vessel that is evacuated continuously from the big pump in the rear of the car. It is not completely sealed or leak proof and as is well understood, brake fluid is not nice to paint and metal so the normal small leak paths, the action of the brake fluid and the physical forces for vacuum and pedal apply open up small leak paths. This normally not an issue to the function of the booster because the air pump is pretty substantial (3.0 L in volume for the SC).

Not to others... when your brake fluid goes away and you can't figure out where it went - look in the booster before it leaks on your Gucci's.

Thank you, you explained the possible leak path well. The seal between the master cylinder and the brake booster must be good enough to keep a vacuum inside the brake booster but not good enough to keep brake fluid from leaking through, even if the brake system works perfectly. That must be the explanation in my case.