At one time, GM had problems with the rear backing plates rotting out on both the A/G bodies, and some of the earlier S/T trucks. Some of these vehicles were subject to a recall to repair this problem. The backing plates would rot out, and the rear wheel cylinders would rotate on the backing plate, then fail & leak. The rear wheel cylinders are not bolted to the backing plates on these cars, but instead are held on by a metal retaining clip. Also, on this style of wheel cylinder, the bleeder screw is recessed in the back of the wheel cylinder, so if they've been in there for a few years, and there is any buildup of dirt or rust, the bleeder screw could be difficult to loosen or remove on the old wheel cylinder. Hope things work out OK for you.