Need help no brakes

[Noah96]

Ok everyone having brake issues. Its a 1969 C2500 4speed GMC with vacuum boost. I've replaced the back brakes and springs along with 1 bad rear wheel cylinder. On the front I've replaced both wheel cylinders along with the Correct Master cylinder which was bench bled before installing. Now I completely bled the system from farthest corner forward go the air out of the lines. When the engine is not running my pedal feels good but once I start the engine the pedal goes to the floor, but with the truck running and the vacuum hose disconnected the pedal is good when i'm in neutral then once I put it in gear I have no pedal.
Also Ive checked for leaks in my brake line and nothing along with the backing plates no sign of a leak.
Any help would be appreciated I am stumped ask a few local shade tree mechanics and no luck.