Brake Question

[68-BEAST]

Hello, I am new to this site and I am having a terrible time with my brakes. I own a 68 GMC Suburban, 3/4 ton, 4x4, rust magnet. It has been a project for the last 5 years, the brakes have never worked right. They finally went out all together and so I just replaced all the shoes front and back. The wheel cylinders seem like they are in good working order but I can only get 1 of 4 to bleed correctly. The front right bleeds just fine, but I can't get good pressure from the others. I unscrewed the lines fromt the bottom of the porportioning valve under the master cylinder, and I get good pressure there, it spurts brake fluid when pedal applied.

While checking the wheel cylinders, the whole system was pretty much dry. I have worked for a few hours and can't get any pressure, brake pedal just goes to the floor. I have manually adjusted the shoes to be touching the drums. I am not a great mechanic by any means, but I've bled brakes before and never had this much of a problem. Where should I look first????

68-BEAST