Good pedal til car is running

evil 340 said
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
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Ran into this before my problem was the calipers were on the wrong side the bleed screws were on top but there was a small void, and not letting all the air out.Switched calipers around and the bleed screws were at the very top.Bled and problem solved.