Brake lock-up: What failed?
 

Reading another braking thread on the 911 board got me to thinking about my recent brake conversion on my 86.0 928.

Most all parts got swapped in from an '86.5:
-ABS pump and proportioning valve
-ABS brain etc.
-Suspension and 4-piston calipers/rotors

Got everything running great, then while I took everything apart for my intake refresh, I swapped in the '86.5 master cylinder and vacuum booster as well.

On the test drive, the brakes rapidly built up pressure and would not release it - until the engine coolant blew the pressure relief cap.

This was not a mechanical failure like a stuck caliper or collapsed hose (calipers are in perfect condition and all hoses are new) - it was a pressure issue. The brakes were working perfectly before the swap of booster/MC, and in order to limp the car home, we knocked loose the passenger outer bleed nipple - and got a high pressure JET of brake fluid. The car was limped back to the shop without issue after that (with a two-car sheriff's escort, that was awesome - they came across us while we were broken down and slowed traffic around a blind corner for us, and were asking all about the 928. They were really cool about it). Had to stay off the brakes on the way back, though.

What might have failed - the vacuum booster or the MC? What was the cause of failure?

Not sure, but concerned with you about it.

First this question.
I assume you swapped the full suspension on the front, including the hubs that have the right tooth count for the 86.5 brain.
What did you do during swap about tooth count difference on rear axles?

Yup, swapped out the entire front suspension including the hubs. On the rear I installed the 86.5 axles. I mis-read the parts diagrams thinking the sensors were the same - nope. This caused the ABS to engage anytime the brakes were applied. Next on the punch list is swapping out rear sensors. Until then the ABS brain fuse is pulled.

But that's not what caused this latest issue, right?

There was a post on rennlist yesterday, an old one resurected, that spoke to brake lock as a function of adjustment of brake pedal rod. Trying to find it.


Here, no answers, but some troubleshooting http://forums.rennlist.com/rennforums/928-forum/518540-brake-drag-lock.html

Judging from recent posts, its an issue for others.

my guess is the new /old booster is bad and possibly the MC as well,
try unplugging the vacuum port to the booster and drive it

also the flex lines should be swapped out with new SS lines

Nah, that didn't cause it. I only drove it around the block before I went back and looked at the parts diagrams, saw my error, and pulled the fuse. The lock-up failure was actually before I had the ABS up and running - some wiring needed to be repaired at the relay panel and the ABS relay was bad.

I did carefully measure the rod before installation, and checked for adequate free play before road testing. Not saying that wasn't the culprit - but I don't think it was.

Gonna go read that whole thread now, thanks!

Yeah...I know the booster and/or MC failed...established that in the first post - I'm just wondering what the specific internal failure is that causes the brakes to progressively lock up.

Didn't have a chance to try unplugging the vacuum line, as I yanked the booster and MC and swapped the old ones back in the same day.

As stated in the first post, the flex hoses were brand new. Your point is well taken, though, as I have seen another car with very similar symptoms and it was due to internally failed flex hoses.

brake mods
 

WHen I got my 86.5 928S and was getting it prepped to slap the 911s around at D.E. there was a PCA tech article(I think) and they recommended removing A small restrictor at the ABS block. it's in one of the lines, remove it and then thread that line right into the same hole as the threads are the same. I did this and never had any problems. The ABS worked great! As the Corvette guys working the cones at the Sun Fun Corvette challenge at North Air Base years ago. I hit the ABS at 140 + about 250 feet from the first "gate". It was looking bad for the cones, but the ABS made me look like Michael Schumacher as I threaded the cones! I was just steering to hit one, but the ABS taught me what it does, not stop shorter as most people think, it gives you control under braking in a curve.

best of luck on the brake quest.