1989 Dodge Ram 50 Pickup Brakes

I had mushy brarakes when I bought this P/U. So after about 6mos. I decided to bleed the brakes. Started on the the rear passenger side. I barely was able to get any fluid out of the bleeder valve. But then my brakes were about half as good as when I started. So I tried again got fluid out of rear Pass. side, but then my brakes went completely out . Because whe I tried to bleed them when the car was not running and the brakes felt ok when not running, I started the P/U & realized there was a vacuum to it. so this time i tried to bleed them while p/u running but no improvement at all.If u are familiar with the p/u it has a brake line running from the drivers side to the pass side back to drivers side. Am I missing something??? I have Bled the brakes on cars for years & have never come across of what ia happening. Please help me if u can. The car is a great running vehicle. I also took the rear drum off and the cylinder was barely pushing the shoes at all thanks for your help