Brakes locking up in non ABS system

Model T with stock Mustang V8, C4 tranny, Corvette rear end and Corvette 4 wheel disk brakes. Sat for 5 years in 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, locked 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. Using 4 Corvette disk brakes. Problem did not occur prior to rebuilding booster. The setup: brake pedal to master cylinder, 1 line out to booster, boster 1 line out to T connector, T 2 lines out to proportioning valve, proportional 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.

without ABS you will be much more likely to lock the brakes up. Especially with those big old corvette brakes on it. Don't know much about them but those brakes seem like overkill. I know less about Model Ts but I don't think they weigh enough to need brakes as big as those. Are you working on a T bucket or just a restoration?

It turned out that whoever rebuilt the booster did not do it right and /or the mechanic who said he sent it back may not have told the truth. A neighbor suggested bypassing the booster and going directly from master cyclinder to proportioning valve and test it and that the way it has been ever since. The rebuild booster was bad and was causing all 4 wheels to lock up. I probably contributed to the situation by doing a poor job of bleeding the brakes (by my self) and not getting all the air out and I later found that cleaning the rubber cuffs on the caliper wheel cyclinders also helped.
Thanks to all for their suggestions.