Brakes locking up in non ABS system

I just bought a Model T with stock Mustang V8, C4 tranny, Corvette rear end and Corvette 4 wheel disk brakes. It sat for 5 years in a garage. I bought it 6 weeks ago and started replacing old, tired parts. The problem: brake pedal was very hard. After new vacuum line installed, I felt a 2 stage stopping pattern. Stopping improved slightly but after letting up & applying brakes again, brakes hard again. Mechanic removed booster and said there was a hole in the diaphram. Mechanic sent booster to be rebuilt. $386. Now all 4 brakes lockup frequently. Mechanic said change out flex brake lines to wheel. I did rear...no change. I noticed brake fluid very, very dirty.
I changed brake fluid and master cyclinder. No change. I took car back to mechanic, he sent booster back to rebuilder, test done said no problems. Got car back, brakes still locking up. I jacked up car, pushed on pedal locking up brakes, released pressure after master cyclinder, front brakes still locked, released out line from booster, front brakes freed. Locked brakes up again, released pressure after booster, brakes freed again. Conclusion: rebuilt booster is bad? Is that the correct test and result?
This is not an ABS system. Problem did not occur prior to rebuilding booster. The setup: brake pedal to master cylinder, 1 line out to booster, booster has 1 line out to T connector, T valve has 2 lines out to proportioning valve, 3 lines out (2 to front brakes, 1 to rear brakes). Bleeding brakes results in clear fluid.
Any suggestions would be be greatly appreciated.

I seem to remember something about a guy installing power disc brakes on a Nova, and something about where the rod from the master cylinder comes through the firewall didn't have enough play or adjustment(something like that) and was causing the system to build up pressure everytime the brakes were applied until they would eventually lock up. I converted my Nova to discs as well, but mine had power drums to begin with so I didn' have to get into fooling around with the booster.This is only someting that I heard though, but it kinda sounds like the problem you're having. :frown:

Three mechanics actually tested the rod tightness and one mechanic adjusted the length of the push rod into the master cyclinder. It is totally floppy (totally loose). I do not think that is the problem since several of us were aware that it could be a problem and we all checked it.
This is an unusual set up in that 99% of the time cars have the brake pedal push a rod into the booster which is attached to the master cyclinder. NOT HERE! Set up is: pedal rod into master cyclinder, 1 fluid line into booster, 1 fluid line out of booster to T splitter, T splitter has 2 lines out to proportional valve (1 front, 1 back), proportional valve has 3 lines out (2 to the front, 1 to the back).
The booster is currently at a 2nd local rebuilder shop. I do hope this solves the problem. What I'm looking for is confirmation that I was right in the first posting or if when I get the booster back and the system still locks up, the correct way to test the system to isolate the "locking or pressure retaining" part or dirty stuck valve or ????
If I get the booster back and the system still locks up, I will try to extend the pedal rod 10 inches to a position where I can install a new booster-2 chamber master cyclinder unit (current master cyclinder has only 1 chamber). I have 2 photos that pretty well show the position and connections if that will help. I tried to load the 2 photos but it is denying my effort. Sorry. I can email the photos directly to you if you wish.