'91 Navajo brake problem

[kdgauger]

I have replaced the pads and shoes on this truck. Also the brake master cylinder. And I have bled at all 4 wheels multiple times. If the truck has been sitting, the brake pedal is good before starting the engine. Upon cranking the truck, the brake pedals sinks almost to the floor. This truck has rear ABS. I suspect the power boost but I am a beginner. Any suggestions?

 

[coco944x4s]

Pull the hose off the booster and plug the hose with something to prevent a major vacuum leak. Start the truck and repeat your brake pedal pushing to see if it still happens. If it no longer goes to the floor, chances are your booster is shot.