As for something preventing brake fluid from returning, it is a slim possibility that you need to replace the flex line going from frame to the differential. These can deteriorate from inside out and I suppose it is possible a breakdown in that line or somewhere else in the rear line is preventing the forward/backward flow of brake fluid. There is only one line going from up front to the rear axle and only one flex tube in the rear and it would affect both sides of the axle. But that is sort of a long shot. As mentioned above, don't press the pedal until the drums are back on and the shoes are reasonably adjusted. If you have the proper shoes and you've installed them correctly, adjusted them and the anchor pins aren't just flat out ruined (another pretty slim possibility in my experience), your shoes shouldn't be able to do this...that is to say there should not be enough room in there to allow movement like this.
