Really strange problem

[wmhjr]

My buddy has a custom '53 Ford truck. G Body front sub, tilting front clip, sbc powered. The brakes are different. Master and booster are '77 Granada. Rotors and calipers up front are '77 Monte Carlo. Everything used to work fine.

He had some issues which resulted in having to replace the master cylinder. Now, when the engine is not running, the brakes are bled and pedal is extremely firm. Can't get any more air out of the system. But when you start the engine, pedal goes almost to the floor when you step on it before producing any effort. Again, lines have been bled more times than I can count. Have tried several new master cylinders. Replaced flex lines and calipers. Old combo valve is gone, replaced with prop valve in front of rear brakes (drums). Booster has been inspected and tested by brake shop.

Any ideas?