You are bleeding correctly and you are correct that the rear system should not effect the front as they are separate hydraulic systems for safety. Bench bleeding the MC again can't hurt. There is a hold off valve on the end of the proportioning valve that must be pulled out and held while pressure bleeding. I understand that you are not pressure bleeding but at this point you have to be ready to try anything, I know I would be. Try pulling that valve stem out and secure in the out position before you bleed again.

Since there were no lines to the rear before you started, exactly where was the system capped off and why if you know? The MC on your truck is of the cast iron dual chamber type, correct? I ask because in 74 & 75 they did use 3 types of MC's and yours should be the cast iron dual chamber.

 

Yes it is a cast iron 2 chamber MC. I'll give the hold off valve a shot, I'll try anything. The system wasn't capped, there was about a foot long section behind the gas tank that was missing (looks like it just rusted out). It's weird because both the front and back reserviors were full on the MC, even though the rear one had a gapping hole in the line. I'll try bleeding it again and tell you how it goes. Thanks for the help.