Because the rear cylinders don't have rubber boots the pistons are exposed to the air and tend to corrode to the cylinder walls. The fact one shoe was worn more than the other could indicate at least 1 is stuck. Maybe all by now.
The good news is that the corrosion is usually near the ends of the cylinder and not in the area that the rubber pistons will ever travel. They can be rebuilt nicely. The rubber cups are easy to find.
I just did a set. PM and I can give you some info on saving them.
