hi, the rear brake lines are steel as they run along the axle, but, in the middle, towards pass side, is a flexible rubber line that connects to the steel line going to the front of the car. It is ALWAYS a good idea to replace the soft lines, both fronts and the one rear, if they are over 30 years old. They can deteriorate from the inside, and let little chunks of rubber clog your calipers/pistons, causing your brakes to lock up. This happened to me once , years ago, though not on an AMC. As for rear pistons, get a name brand, don't go cheap. Personally, I like NAPA parts, but others are ok as well.
As for the MC failing altogether, yes it can, though it's designed not to. Years ago, I had a MC fail COMPLETELY in an 82 Concord. Just by pure luck, the car was parked at the time in a Home Depot parking lot. Drove there fine, got back in, started out, and NO BRAKES whatsoever, had to walk home and get a tow. Could have been a lot worse, 5 minutes later I would have been on the interstate.
As for your high idle, your choke is sticking, before you adjust idle screws, make sure choke pull off is good.
good luck, gz