I have a 65 T-Bird with power disc brakes. (drums in rear) Everything is new except the power booster and proportion valve. With the engine running the brake pedal will not return to the released position after depressing it to stop. I must lift it back up with my foot. (And the brakes remain on until I manually lift the pedal). However, with the engine off the brake pedal will return on its own. Any help is greatly appreciated. Many thanks!

 

Check for a plugged vent/breather on the booster.