Back Brakes won't bleed

I was just checking years, and most likely you don't have this - a combination valve. I think in 71 you may have 2 blocks. One for the switch and on for propotioning disc/drum. Anyway here is a pic of the bottom end of this type opened. It will have spring pressuure on it when the cap is backed off. Note the black rubber piece around the brass piston - that is actually what is wrong with this one. They swell up and fall apart. There was a supplier for a kit which I believe included these(Muscle Car Research) , but there website won't come up now. Without that, the only thing you can do is clean them (flush with clean fluid). There is nothing in them to 'fix'.
This is the proportioning side of the valve. The 'slider' that trips the light and may be your problem is actually on the other side of the valve. So, you can't clean that directly without complete dissasembly. If you get flow through the system that side will flush. So what I said about the slider sticking would probably only happen if the system was dirty and out of service for a while (like mine was).