Brake help! - odd problem (as usual for me)

Hey guys,,, I posted this some time ago I think, over a year,,, but here is the background as we have not been able to sort it out yet.

1983s, early VIN 1108
Symptoms -
1 - For over a year or more, my brakes have been releasing slowly. Approx 2-3 seconds before fully released.
2 - I have also driven in the 95+ degee heat for a little while and found the problem sometimes worsens, to the point that after a full stop and trying to hit the gas,,, the car would not move,,, brakes fully engaged without my foot on the pedal, had to wait 6-7 second it seems before I could move,,,, then took a few hundred feet to fully disengage ,,, could tell there was drag while driving for a little while. - This condition has only happened once howeve,r to this extreme
3 - The car does NOT pull or brake unevenly.... ever. When the brakes release, it seems like all are releasing the same.
4 - My mechanic has 1 928 and 3 944s and does brakes and suspension for a living and this one has him stumped right now, he has not seen it before.
5 - The harder you brake, especially to a full stop,,, the longer it takes to release. Light pedal = quicker release, but always a little lag. This started a little over a year ago and has acted the same way ever since, no better, no worse. I have driven around VIR 2.5 laps with the Porsche parade last August and ran for 2 straight hours on a fun run at SITM this year in the mountains, with it doing the same thing the whole time. Again no better, no worse. Braking is not the issue, only releasing.

Equipment -
1 - Current Brake Master Cylinder was new in 2011, still seems to be in good shape, but I do have a new one to use if needed, if I do install it and it does not fix it however, I am out a couple hundred bucks - so I am trying to understand before replacing more parts. I understand that this could end up being the culprit if there is seal damage etc. So I am not writing off changing it out.
2 - The 2 brake pressure regulators, under the Master, used to control front to back pressure, were swapped out for a re-conditioned/used pair. That one we did just throw money at.. But no change afterwards. It was relatively inexpensive.
3 - New SS braided brake lines were installed a year ago
4 - DOT 4 Super Blue ATE fluid is being used. Flushing and filling have been done and went fine,, fluid moves in and out with no issues. Right now we do not suspect clogs or line issues.

Testing -
1 - The issue only appears when the engine is running, brake system charged.
2 - We did try this test, which to me, seems to indicate it is not stuck cailpers, pad issues or something mechanical like that.
- Car in the air on lift, wheels spinning free - engine OFF - hit brake, then release and another person turns the wheel. Seems to work normally and release
as soon as the brake pedal is released! Every wheel releases normally with the engine off. There is no problem with the engine off it seems.
- Car in the air as above, but with engine running - same sequence for every wheel,,, every wheel releases slowly, 2-3 seconds.
3 - My Mechanic attests that the wheel, brake hardware all appear to be normal and in good shape, rotor wear, pads are new etc, bleeding went fine etc.

So after looking at a few other posts elsewhere, it is starting to sound like something is not turning of the booster quickly or the master cylinder is not opening all the way etc. (Wally I saw one of your posts referring to pedal settings and how the master is working, that helped get me to this point)

So there you go guys - have at it! HELP!

Brake booster check valve and/or vacuum line to check valve issue? Unless of course I'm interpreting the info at the following link backwards?

www.freeasestudyguides.com/brake-booster-operation.html

Sounds like engine idle vacuum is holding the master cylinder side of the booster too much as you suspected?

Yep, this is kind of where I was going with this myself, almost isn't too much left it could be given the current symptoms.

Just wanted to get some feedback to see what others say this weekend before we made further plans.

Thanks for the link, I will check it out.

Well I'm by no means a brake system expert, so please take my suggestion with a grain of salt. Based on the other symptoms/possible causes listed for brakes slow to release at that link, the only other possibilities were things like brake pedal lever return springs and brake pedal linkages binding, which I suppose could still be the cause, but I'd imagined you'd already looked for the simple stuff like that.

The return spring INSIDE the brake booster was also listed, but if it turns out to be the brake booster, wouldn't the entire piece have to be replaced anyway?

Good thoughts, but I am leaning away form pedal settings and springs etc because it works just fine with engine off. That is why I am thinking it is boost release related. Keep 'em coming though thanks

Ok - so here we are (Where are you Wally?

- problem still occuring
- MC replaced - to remove possible bad MC and seal issue there
- Calipers rebuilt - was about time anyway and remove seal issue as well
- Repeated an earlier test, which was done incorrectly the first time,,,,,- pulled vacuum line out of the booster, capped it off ,,, went for a drive, no drag whatsoever! Release is normal. Just no boost as expected.

This puts me back to to the idea that it is keeping boost up too high when you let off. So unless there is some other regulate or valve that could do that, I am thinking the booster must be the culprit (we have pretty much taken care of all the other parts!

Best suggestion now - new booster? Is that difficult to replace,,, it looks tight in there (83)

Thanks