Pedal goes to the floor... And i am stumped

So I have been chasing a floored brake pedal issue in the Shelby Dakota. Pedal went to the floor in the driveway - lucky me. Barely enough brake to hold it from rolling at idle. No leaks, master is full. Assumed bad master. Bought a new one. Bench bled, then bled at all 4 wheels until nothing but new fluid came out. Good solid, bigh, hard pedal..... Until i start the truck. Then it goes to the floor with very little effort. Bled em out again. No better. The pedal is dang near to the floor when depressed, with little effort. Can hear and feel the brakes working at all 4 corners, but i have never had a pedal so low. Thought maybe the power booster was failing, so bought a new one. Same issue.

Thoughts?