Did I 'F' it up? (brake booster issue)

[Josh D]

So Saturday I got around to replacing the leaky brake master cylinder. After getting everything out, the booster was full of fluid (as I suspected). I drained the fluid the best I could and cleaned out all the gunk in the solvent sink.

I got everything back together, brakes bled, and test drove it, putting on about 12 miles. Everything worked fine. Brake pedal feel was better then before, good bite, etc.

I didn't drive it at all on Sunday. This morning, I back out of garage and didn't notice a problem. However, by the end of my street I lost power brake function and it's clear I have a vacuum leak when I press the pedal. I drove it a couple miles before doubling back and taking it home.

Brake pedal is hard, vacuum leak causing idle stumble, and I can hear air/leak at the booster/MC with foot on the brake.

Any ideas on things to check? Can the booster be dissasembled and checked for damaged diaphragm or seals? Could this be an issue with the check valve? Did the solvent from the parts washer damage the seals/diaphragm?

[chrisbruck]

There's an "O"-ring between the MC and booster. Did you replace that? I forgot mine once causing a similar headache.

There is only one other vacuum hose connected to the booster. Make sure it's snug.

Not sure on the internals to the booster though.

[Josh D]

Originally Posted by chrisbruck

There's an "O"-ring between the MC and booster. Did you replace that? I forgot mine once causing a similar headache.

There is only one other vacuum hose connected to the booster. Make sure it's snug.

Not sure on the internals to the booster though.

Yes, replaced O ring and was careful not to over torque.

I moved the vacuum line and check valve around while my wife held the brake pedal down to see if that affected the leak rate, but it made no difference. I would think if the vacuum line or check valve connection was the problem, the leak would be continuous. The vacuum leak only occurs when brake pedal is depressed.

[Bob Kontak]

See post #5 in thread below. RoninLB mentions that one side of the diaphragm opens to atmosphere when pedal is depressed. Maybe that is what you are hearing?

Also some other ideas for checking stuff. Hope it's just something silly.

Parts washer? Was it submerged?

Brake booster testing

[chrisbruck]

Sounds like you checked the basics.... must be something internal.

[uwanna]

Hate to be the bearer of bad news, but I had a very similar problem to what you had. I replaced the MC and found the booster full of fluid. Cleaned it up and put it all back together and it was AOK for a while, but then the booster failed. I think the brake fluid may deteriorate the diaphragm in the booster.
Just keep your eye out on the for sale threads, as good used ones come up regularly.
Do a search, as I saw some just last week.

EDIT: you might try this fella.

FS: 911 Brake Booster

[Josh D]

Thanks for all the replies. The MC had been leaking for awhile and the internals of the booster were pretty nasty. I decided rather than wasting good time trouble shooting a 35 yr old part only to come the the conclusion that it has failed, I just ordered a reman booster online. Wasn't expensive at all.

The next challenge will be to try and remove it without removing MC. I'd rather not disconnect brake lines and have to bleed again. Makes a frickin' mess!

[billh1963]

Originally Posted by uwanna

I think the brake fluid may deteriorate the diaphragm in the booster.

Brake fluid will absolutely destroy brake booster diaphragms.

[sveiks]

Hey Josh,
I'm looking for a new booster for my 79SC.
Where did you order yours from ? Link ?
Thanks

[uwanna]

Originally Posted by sveiks

Hey Josh,
I'm looking for a new booster for my 79SC.
Where did you order yours from ? Link ?
Thanks

Ran across a post by Tidybouy with a link to his Ebay auctions. He has a real nice booster and MC set for sale. I am following (just for fun,not a buyer) Only one bid at $75, couple of days to go. It looks real nice. PP link:FS: 3-Pin CDI, brake Booster, CIS Air Box, Bosch Fuel Pressure Tool, much More


ebay link:Porsche 911 Power Brake Booster Master Cylinder 911 SC Carrera Master Cylinder | eBay

In my earlier post in this thread I said I had the same problem you have. Well I bought a similar used booster and MC from a Pelican, and it's been great for the last 5 years! Maybe your solution. Maybe PM Tidybouy, and ask him about condition etc. Good luck!