Brake bias?

I am having the same problem. I am using the factory distribution block/prop valve, whatever you want to call it. When I try to bleed the brakes, nothing gets through on the rear side of the block. The little piston/spring thing is not actuating. The way I understand it, the drums need to be preloaded and take more pressure to keep up. The little piston, spring thing in the prop valve is what makes this happen. I am thinking of gutting it (the rear part) and putting an aftermarket check valve in-line on the rear, which dodge freak suggested above.