As a sanity check - check the brake caliper piston cylinders (not sure of the correct technical term)..... On an 85, I had scored cylinders and they were not releasing - the pistons were hanging on scored cylinders - wouldn't release until they were cooled down. This may not be your problem, just worth checking....

 

My point was that if you did not remove the check valve from the booster then your test of the booster was fouled, because the vacuum was not released properly. This would be confirmed by the hissing sound when you removed the master cylinder from the booster. How far from Leeds Ave / 526 are you?

 

Ahh, I get you now. When I had the brakes locked up I pulled the check valve out of the booster completly and the brakes were still locked up, pulling back on the pedal released them. So I put the vacumn line back in and drove home.

After that I put the old MC on and I couldn't get a hard pedal with the eninge runnng, the pedal went to the floor, so I pulled it off again. I also pulled the plunger part of the new MC out hoping to find some obstructions or anything and didn't see much of anything. I think doing that killed that MC now since I can't get a hard pedal with the engine running with the new one, or the booster is now dead. It seems like the vacumn is being applied inside the MC shaft. I can seperate the MC from the booster and not hear the vacumn release, but once I pull on the MC a little I hear the vacumn release once the MC shaft comes out and the rubber seal actually gets sucked into the booster at that point.

I'm going to get another MC this once has a lifetime warranty on it, it takes a week to get though. Just not sure about this booster now, the pedal is hard without it and goes to the floor with it, so maybe it's ok.

I'm on Johns Island, so probably about 15 miles from Leeds.

 

Did you bench bleed the MC before installation?

 

Extra bench bleeding was done on the new one for this last attempt. It was clear of bubbles from what I could see. I'm getting really good and MC removal and install, I'm leaving the reservoir attached now. I think I'm down to 10 minutes for removal/install.

 

Changing the MC just doesn't cause problems like this. It may have accentuated an existing problem. You old MC probably has worn seals and thats why it feels spongey...ultimately never getting firm.

If you bled the MC correctly, which is no complicated nor difficult task, then this issue seems to be pointing to your booster. To my limited knowledge, there is no way and not worth it to mess with trying to fix the booster. You can find them for $50 to $100 on Ebay or here on Rennlist. If you had one wheel locking up than I would say you had an isolated issue at the wheel, but you don't. The brake system isn't really all that complicated.

With all that you've tested, the brake booster seems to be the issue, so a replacement is in order and then you can move on with your 928 life. JMHO

 

I may just order the booster, I need the car drivable by the 29th. The car is suppose to be showing in one of the car show displays at the Hilton Head Concours. So once I get the new MC and Booster this problem should go away or this car is just cursed. This all started with simple maintainance of flushing the brake fluid and changing the old rubber hoses.

 

Got a new MC on there today, not a rebuilt one. The pedal isn't as hard as the rebuilt one got, perhaps that was part of the issue. But it does still stop well and will lock up the brakes when pressed. But the brakes do not lockup on their own which is a nice feature of this new MC. I'm thinking perhaps a new vacumn check valve might help the pedal feel, or it's the pads the front pads were getting thin before all this brake dragging started so I'm sure there even worse now.