Power Drum Brakes: Pedal Goes to Floor?

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?

Bled to no air bubbles..but was the pedal hard and steady?

Yes, after bleeding, pedal was hard and steady...until I start the engine.

air gap between master and booster?, should be .020 to .060 max, too much and pedal goes to the floor...

I'll check it out, THX!

an engine valvetrain pushrod makes an easy tool to measure the distance out to the MC flange then you want the booster pushrod .020" less than that distance protruding out from the booster flange.

if you make the pushrod too long, brakes will be applied anytime the engine starts