Rear brake problems

[xtreme80]

When I put my new rear end in the truck, I used the brakes on that one, instead of using my old ones (mine were trashed anyways). We bled the brakes, they worked great, and the next day they didn't work again. So we bled them again, and the very next day they didn't work again.

I've been watching the fluid level, and it's not getting low. There's also no leaks anywhere in the brake system. I was told that it may be possible for air to be getting in the rear lines or bleeders without fluid actually coming out, but I'm not sure how true this could be.

The only other thing I can think of, is a bad proportioning valve. But it seems odd that the valve would go bad right when the rear end was put in.

Anyone have any ideas?