B2000 Brake Problem

My B2000 will not hold pressure on the brakes. Every time I use them I have to pump them for them to stop the truck. I just put on rear brake cylinders, a junk yard booster and master cylinder. I have bled the brakes several time with no change. :banghead:The hose off the booster, does it have some sort of check valve in it to maintain pressure? Any one heard of the proportioning valve going bad, and where would I get one? Thanks