Use clean brake fluid or brake clean to clean the caliper and
Piston, then wipe the caliper and piston with lint-free towels or low air pressure. Don't add solvents such as gasoline, kerosene, thinner or others, since they might bring residues that harm the piston and seal. Examine the piston which comes from phenolic resin, making sure it is undamaged and clean. If it shows cracks or scratches, put in a new one and do not try to improve a damaged surface with sanding. When you replace the caliper piston, make sure you use the same type, as phenolic resin pistons and steel ones have differences in pistons, seals, seal grooves, caliper bore and piston tolerances. Rust, corrosion or signs of excessive scoring can make the bore unsafe. If the bore isn't too badly damaged and up to 0.025 mm (0.001 inch) wider after polishing, no parts are needed, but if heavy damage requires more work, change the caliper.