Help with brakes 62 c10!!

[Bclanton]

Hey guys Im in need of some help. I have a 62 c10 with brake issues. I have replaced the MC and booster along with the rear brake hose. I have bled the system and I get pressure at the pedal with the engine off but with the engine running I have no pedal. We tried to bleed the system with the engine running and the rear wheel cylinder has very little fluid coming out. It almost seems as if the MC isnt being depressed enough. Does anyone have any suggestions?

Thanks
Brandon