Rebuilding 81 KZ750H Calipers

I am currently putting together an 81 ZK750H and I'm looking for some advise on rebuilding the existing caliper(s). The calipers and one of the pistons show some rust pitting (possible evidence of moisture in the fluid?). Would rebuilding the caliper with this amount pitting be acceptable, inadvisable or would a rebuild kit make them useable? Below are photos of the rear caliper and piston showing the worst of pitting. NOTE: the front calipers showed some pitting (not as severe as the photos) and none on the pistons.