No brakes when I start engine!!!!!

Hi everyone. First time user here. I hope someone can help. I have a 1991 Nissan Pick-up 4x4. When the engine is off, I have a very good hard brake pedal. When I crank the engine, the pedal goes to the floor and I have to pump the brakes several times for them to work. I have replaced front brake pads, rear brake shoes, new rear wheel cylinders, master cylinder, and bled the brakes several times. The problem still exists. The front calipers are both good. There is not a leak in the system anywhere and the check valve on my brake booster is good. I don't know what else to do. Please help. It is greatly appreciated. :banghead:

im gonna take a guess and say vacuum lines? have u checked all of them