"Recall" of brake booster kit component and solution to problem
to brake binding problem.
To anyone who has bought the brake booster rebuild kit but not yet installed it, don't. To those who have installed it, I have figured out the problem with the brakes binding and will work with you to correct it. Anyone who has gotten the kit please get back in touch with me. This only pertains to the replacement seal "cone"; all the other components are fine.
The problem is that the inner diameter of the aluminum cone piece tapers just a bit too much in the narrow end and prevents the end of the rear piston of the master cylinder from freely fitting all the way down inside of it. Thus, when you bolt the booster to the MC, the MC pistons are pressed a few mm forward and are unable to return fully.
There are a couple possible approaches to fixing it. One could bevel the nose of the MC piston a bit so it would clear the cone. It has no structural function per se. You could even cut about 6mm of it off completely; I think this would work perfectly, but it's probably better to keep the MC piston shaft within the bore of the cone for its full travel. That way it cannot possibly bind if it wobbles a little on its return stroke. The correct solution, of course, is to re-machine the aluminum cones, which will start immediately. All buyers will automatically get a new cone; keep the old one for desk art 8^)
If anyone wants to correct their own cone, all that's needed is a light over-bore of about the last 7mm of the inside narrow end of the cone. The diameter of the MC rear piston extension is 25/32, and the diameter of the cone is 23/32 at the step for the seal, so a 51/64 drill bit with a light hand would be plenty. You could easily do it by feel with a Dremel tool as well. But again, everyone will automatically receive a re-machined cone. (Interestingly, I'm quoting SAE measurements because many of the internal components are sized that way. The actuating rod is 3/8", the seal is 7/8", etc. Apparently it's still standard practice in the braking business to use the SAE standards).
My apology to all for the accidental oversight. I did the test fitting on a master cylinder that had been disassembled, so this issue regrettably didn't come to light. I regret the turmoil it surely caused with the brakes locking up after assembly, and assure you that I will work with everyone to remedy the problem.
(Tip of the hat to Wuffer, whose observation of a balky fitment issue ultimately led to the realization of the problem).
Re: Recall of brake booster kit component and solution to problem
I believe that I've contacted everyone who bought a booster kit with the replacement cone, but if I missed anyone please get in touch. I now have cones on hand with the corrected machining and will swap them all starting with tomorrow's mail.
FWIW, part of the reason I missed this issue during prototype testing is that the master cylinder from the e32 750il has a much smaller diameter rear piston, and that's what I was using; clearance would never be a problem on the 7 series cars. It might not be a problem on every 8; there seems to be some variation in the internal components that ATE uses in their brake components. Nevertheless, all the cones will now be set up to accommodate any known cylinder configuration out there.