Good pedal til car is running

72 demon
73 power disc front/drum rears
All new lines. New prop valve
Reman master (bench bled)and booster/booster holds vacuum when car is shut down and u pull check valve it lets out air
Check valve works one way
Good fluid to all 4 corners.
Decent pressure when not running, but as soon as it starts .....straight to the floor


POINT ME TO THE PROBLEM please. Thanks in advance