I believe that this has already been mentioned, but it may solve all of your trouble. Even if it doesn't, it is a step one needs to make before bleeding the brakes.
1. Take all of the drums off and visually check all wheel cylinders for fluid leaks.
2. Tighten the brake shoes until you can barely move the wheels by hand. (You will need to back the shoes off as per The Book after you are done bleeding the brakes.)
If the shoes are too loose, the wheel cylinders may use up all of the volume that the master cylinder can deliver before the shoes contact the drums, thus there won't be enough pressure left to push out all of the air.
