when you say you replaced the back lineS, I take it you mean the hard lines on the axle housing. But what about the rubber flex line from frame to axle tee fitting?. Those can deteriorate internally and act like a one way check valve. It usually happens at a strain point. I've never seen it on a rear axle flex line, but I've seen it twice on front brakes on Darts about 20-25 yrs old. Your Duster is almost 50yrs old.