Rear Drum Brake Issue

[saskman02]

My 67 C-10 was converted over by the PO to a 1978 5-lug front disc / rear drum setup. The issue I am having is only the top of the rear brake shoes are contacting the drum, the bottom 2" - 3" of the shoes look like they just came out of the box. I took it out to a gravel road and all the braking is being done by the front discs, I could not get the rears to lock up. The truck was fully restored 5 years ago and when I pulled the drum off for the first time there was little to no brake dust telling me the brakes have never worked properly since the restoration. All components appear to be have been replaced 5 years ago. I bled the rears and no air was present in the lines. Also turned out the self adjuster until the shoes were just starting to contact the drums and still the bottoms won't bite when braking. The drums measure 11 5/32" and the shoes are 2 3/4" wide. The booster & master cylinder are both new. The proportioning valve is rusty so it looks like it came off the 78. As shown in the picture below, the portion on the shoes that are contacting are a slightly different color than the portions that aren't. I wish I had measured the difference in thickness of the shoe from the top to the bottom but I forgot. Both sides have the same problem and are wearing almost exactly the same. Any ideas as to why the bottom of the shoes aren't biting??