For years, we have been advising customers that they need not replace their brake caliper pistons and consequently we did not carry them in our inventory. Our understanding was that the original pistons are composed of a chrome titanium steel alloy which is extremely strong and hard and will not wear or corrode. Instead of replacement, the surface discoloration or apparent pitting can be polished out with number 1200 emery paper.
A jammed or locked piston may not need to be replaced. Pistons locked up because the seals are bad causing brake fluid and debris to blow into the piston chamber and gradually coat the walls with residues. Clean and polish the chamber walls then install the new seals that come with most brake caliper rebuild kits which may be all that is needed.
But when brake caliper pistons absolutely must be replaced, we now stock replacements for: