Bremskraftverstärker undicht!

[Erich]

Subject: Re: H31 brake booster rebuild kit and overhaul info
Author: chromeowner : member since July, 2007
Posted on: 2011-06-30


Just a couple quick notes on the booster kits; first, I need to make everyone aware that there is a defect in one of the seals in the booster rebuild kit. (This might not apply to E32s, but definitely to e31s). It's the one that goes in the rear position and seals against fluid going into the passenger compartment, over the brake pedal. When I had the kit made up, I sent an entire booster to the seal manufacturer. They recommended a polyurethane rear seal to replace the original rubber one. There is a chance that they can leak, even when installed well; their engineers made a mistake, and it would seem that poly is not a good choice for this application. If your existing rear seal is good and doesn't leak now, don't touch it; just leave it alone and use it again. All the other seals and o-rings are fine and work well; just don't go near the rear seal.

I'm having some NBR seals made up, but it's a slow process because a mold must be made. There are no other substitute seals nor any options besides buying a new cylinder (which, of course, is why I made the kits in the first place). I apologize for any inconvenience this has caused; in the future I will ask for multiple opinions on ANY engineering question.

Second, ATE/Continental may have made running changes to the boosters. I've encountered them with different-sized imput shafts, and there can be minor variations in the ends of the output shaft. Be careful if you're swapping parts between different units.

Third, the input shaft on certain e32-spec cylinders is 7/8" diameter. The ones I had made were all 5/8", so there never was a full replacement kit for certain cylinders. The early type seals I've seen for the 7/8" shafts were a 2-piece setup of a square profile o-riing and a nylon backing washer. Those might be easier to source than the custom one-piece seal used in the 5/8" units.

All of the other internal seals work in all the units I've found so far, and as long as your input seal is intact you can benefit by rebuilding the rest of the unit.
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