Started getting a wear indication noise from my single piston rear brakes, only in reverse which seemed weird. Both rear passenger pads were completely gone while driver side had at least 50% remaining. Discovered that the passenger side piston boot is torn and the piston is frozen in place. Assuming I am replacing the caliper and not rebuilding it can I simply replace the bad one or should I replace both rear calipers at the same time?

 

Typically, brake hardware is replaced in pairs (both sides of the same axle). However, you should be fine just replacing the frozen caliper as long as the other caliper is in good, working order. Just make sure your rotors are in good shape (have them turned (refinished) or replaced) and install new brake pads for both sides.