Brakes don't want to bleed why?

My passenger side slave cylinder piston came all the way out and I lost almost all fluid and brakes. I replaced pads, parts and slave cylinders on both sides adjusted the pads etc. Now I', trying to bleed the brakes and I can't get the brake peddle to depress until I have power on in the truck. Even when I have power on and the peddle moves I only get a small amount of brake fluid to bleed out the back, nowhere near the amount that should bleed through. It's not the master cylinder I checked that and it's OK, and since the brakes bled almost all they way out when I first lost them I don't think it's a clogged line. So what is holding the brake fluid up? Is it what my manual calls the proportioning valve?