Brake issues

[CobraDBlade]

I have a stock 1990 Supra Turbo with a brake issue that is driving me insane. The brakes work fine when the car has been first started up, but after about 20-30 minutes of driving, the pedal goes almost completely to the floor before the brakes apply. The car had a brake fluid leak at the right front caliper and between the master cylinder and brake booster when I bought the car a year ago. MC, caliper, and booster have all been replaced (booster was almost half way full of fluid!). I made sure to bleed the brakes each time a line had to be removed, but am scheduling a power bleed/ brake fluid flush just to be sure. What's really weird though is that when the pedal starts dropping, if you park the car, pull the booster vacuum line (or otherwise vent the vacuum out of the booster) then the brakes will work fine for another 20-30 minutes. Could the booster check valve be the issue? If not, what could? Any help at all will be much appreciated.