Wow - I just got home from a trip to find this post is not dead. The only reason I even suggested the distribution block is because I had an experience like Joe's many years ago on a Chevelle (which I'm sure had a virtually identical design). I replaced the shoes, bled the brakes, then bled the other circuit and was without brakes. I couldn't get the brake light to go out either. Someone wiser than me suggested the distribution block and indeed that was the problem. In my case, getting the piston to recenter solved the problem, but only after a week of trying other things and a gallon of brake fluid.

I actually think Dennis has a plausible theory. The piston moved far enough to one end to lose the o-ring seal, thus allowing air to enter the line from around the switch threads. Surely you can agree that this theory is plausible Jim because you suggested that the same problem was occuring at the front wheel cylinders. Similarly, the o-ring could've gotten cut when it moved over some crud that could've easily built up over so many years hence causing the same problem.