Rear Caliper piston rusted

[qprint]

The rear pads are worn, therefore the piston on each caliper is extended out and I see rust on the piston and I'm afraid when I push the piston back into its bore [to make clearance for the new pads], the calpier might not work as well, cause I pushed that rust and dirt into the bore. Can I sand that rust off in a round rotation or sand from bore to outwards direction ?

[marc780]

Probly worth a try - hopefully that rust is on the outside looking in and not indicating a rusted piston bore.
Here is what i would do, use steel wool or emery clothe and sand the exposed portion, use care not to abrade the seal. Clean it well with break cleaner, blow it off with compressed air and lube the exposed piston and seal with brake fluid. then push the piston back in and install your pads, keep a close eye for brake leakage for awhile after that.
If the calipers leak after that, i wouldnt bother trying to rebuild them - just replace them.