What you have to worry about is corrosion on the pistons, and the cylinder bores. This is where moisture collects in the caliper. When you rebuild the calipers. See if there is any pitting on the pistons and a build-up of a white material behind the pistons in the bore. If the pistons have pitting, get new pistons. The white material can be scraped out with a screwdriver/pick. Then you want to hone the bore. If the pitting in the cylinder walls is bad. Get new calipers. Where there is pitting. This is where the corrosion will start again after a rebuild.
I've done a few calipers in my time.
