86 Front brakes locking after MC rebuild
 

86 928 Vin 860677
I had the rear brakes lock up and had the brake master sent out to be rebuilt. I was told the rebuild seals were not available for my year so decided to send it out.
Upon return the rear was fine but the front was now locking up. I returned the mc and was told the spring was installed backwards. Installed and front was still locking up. Upon removal of the Mc I found the C-clip came off the back and sent it out again.
I have the same problem fronts are stuck.
My troubleshooting consists of the following:
Drivers front caliper pressure released by opening bleed valve, that freed both fronts
on applying the brakes same outcome locked up.
Removed brake rod to check for preload still locked up.
Unbolted the Master from booster and still locked up.
My understanding is that the master rest position is not allowing air to return to the reservoir, pedal is rock hard. The Abs has been disabled for years and I did check the proportioning valve for any sediment.

I have not found any replacement apparently my vin 60677 is a split year. Any feedback if I’m missing something, a reference on another rebuilder or an availability of a used one will be greatly appreciated.

Is this one of the failure modes when the spring is not the correct length? @Mrmerlin has a lot of information on here about what needs to be done to (certain?) master cylinders if they are rebuilt. Perhaps it's worth seeing if you can find some of his posts on the issue.

Good luck

For clutch masters, not brake masters.

@GregBBRD : Doh! Thanks for the clarification.

If you think your issue is the master and not the booster,
I'd be looking for another rebuilder, also.
Actually, once they told me that they had assembled it with a spring backwards, I would have been done.

That whole "brake system" stuff needs to be fairly proper.

Open to another rebuilder if anyone has a recommendation.

Buy from reputable 928 suppliers like 928 International and 928sRus. We both sell rebuilt Brake Masters.

I believe that 928 International still had a couple of brand new masters, in stock.
If not, I certainly do.

84 to 86 - 928 355 011 23 was dropped back in 2014.

How old are the brake lines? They swell and narrow inside and prevent fluid from passing backwards and the pads from releasing.

Here's a picture of the cracked and hardened rubber lines cut open that I removed from an 81 Euro S compared to the stainless ones I installed.

Look closely as there's more rubber insider the reinforcing braid.

https://cimg5.ibsrv.net/gimg/rennlis...895f3c4ef2.jpg

Absolutely!
We call it: "Hardening of the brake lines", in reference to the old medical diagnosis of: "Hardening of the arteries."
New brake lines are almost mandatory these days.
However, the change from rear brakes locking to front brakes locking, when the master was rebuilt, doesn't scream "brake lines", unless the rears locking up was an incorrect diagnosis.

I have the S4 brakes newer SS lines problem occurred after rebuild.
I was under the assumption that the kit was not available for my year and vin, found a post recommending this company for a rebuilding. I’m already close to $400 into it with shipping charges new is going for the $800 range.

Interesting.
https://cimg0.ibsrv.net/gimg/rennlis...a4bbae41f6.jpg
Master cylinder shelf. Two deep.

Hoarding Porsche parts has a better return in investment than the stock market!:thumbup:

Assuming (which may be incorrect) that the green circled numbers indicate the manufactured date (week.year) that would correlate with Roger's statement.