69 Charger RT, new MC and stock booster/prop valve, replaced front lines during engine rebuild. Bench bled MC, then bled RR, LR, RF, LF several times until no air bubbles (Dot4). Pedal good until engine start at which point pedal goes to floor. Stock engine rebuild so plenty of vacuum for booster.

Did it again, incl bench bleed, same result. No leaks apparent, new wheel cylinders. Would seem to be booster related as that is the only brake sys component activated at startup, although IIRC a bad booster typically causes a hard pedal.

Suggestions?