Brake Help

[greg72]

I just got the truck up and running but I have beld the brake on three seperate occasions. Each time I get almost like a vacuum in the master cylinder where after two bleeds, I get no more fluid from the right rear. I replaced the wheel cylinder and bled this time with the master cylinder with the lid off. Again after three days the brakes got spongy and the side going to the rear brakes was almost empty again. I can see no leaks at all. What about the proportioning valve? Is it possible to have an air lock inside it? Is there a procedure to bleed it or test it? I am stumped.