The main cause of calipers seizing is that brake fluid is hygroscopic, it absorbs moisture.
Since the fluid in the brake system does not circulate, the calipers are like 4 "dead ends", moisture collects in the caliper and gradually the piston seizes.
While the fluid in the master cylinder reservoir looks nice and clean, the fluid down in the calipers becomes contaminated crap.
Bleeding each caliper just a little to get rid of the crappy fluid every couple of years can make the calipers last a lot longer.
Sort of, but not really.
I own a caliper rebuilding business...mainly specialize in air cooled porsche and vintage british, but do everything. Also worked for Centric Parts for about 20 years.
A caliper that has been in use almost never has rust in the wet side of the system. I take apart calipers that have been sitting for 30 years on a car in a barn or a field and they don't have rust in them. Where the issue lies is on the dry side of the seal, between the dust boot and the seal.
Out here on the west coast we don't have these issues...but anywhere you have road salt/brine...it's a problem.
Whatever coating/plating was used on the casting will degrade, and when that happens we'll get rust. That rust holds moisture, much like a sponge. It will eventually creep underneath the dust boot, up the dust boot boss on the casting and start down the bore. When it gets to the bore, it will grab the piston. There's only about .005" of so of diameter difference between the bore and the piston, and it doesn't take much to fill that space with rust.
Cheaper reman calipers don't have any coating on them at all. These suck and should be avoided. Some calipers will have a paint or powdercoating...these should also be avoided, because the critical areas are all masked off before the coating process, and are left as bare iron (bore, under the boots, mating surfaces, etc...
Zinc plated calipers are as good as you're gonna get. Most of the new aftermarket calipers coming out of china (where they all come from) are pretty good and are zinc plated. You can further increase the chances of success by taking one of those little syringes with the curved tip like these
McMaster-Carr
And fill it with silicone dielectric grease...like the stuff they try to sell you when you buy spark plugs (or for the older folks...spark plug wires), then peel the boots back with a pick and squirt the dust boot bellows full of this grease. That will create a boundary seal and help prevent rust.
You might consider getting a can of some high end clearcoat like POR15 aerosol topcoat and spray the whole caliper too...as the paint (clear) will seal the zinc, adding a second line of defense.
I've got the original calipers on my 2003 4runner with 240K on it, but nothing rusts here. lol.
