One more time for the benefit of any new comers willing to measure the posts here against the real world.
In the common single piston caliper, the piston rides in a slightly over size bore, but only touches the seal, a square cross section ring that fits in a groove. When you apply the brake, the piston pushes the inner pad against the rotor, and the caliper slides in and pulls the outer one against the rotor. When you let off, the piston and caliper are pushed back just enough to clear by the wobble of the rotor. There are no springs. If the hardware is in good shape, the piston and caliper both move, but very little. If you do not lube and clean up or replace the hardware when you change pads, the caliper will not slide in as it needs to to pull the outside pad against the rotor. This is a frequent problem causing the inside pad to wear more and uneven braking.
As the pads wear, more and more of the piston stays outside the seal, untouched by the brake fluid. It is protected by a rubber boot, sealing out moisture, grit, salt, etc. Anything that makes it in past the boot can attack the the chrome plated steel piston. The plating is thinned where the it was abraded by the seal. It is slow, but rubber does abrade hard metals. Of course, the seal wears away too. The boot is next to the back side of the pad and heat from it and age harden the rubber and reduce its elasticity. It becomes less and less effective in keeping out yuck, even if it isn't cracked or torn.
Fresh fluid never touches the boot, the vulnerable part of the piston outside the seal, or the sealing surface of the seal. I doubt if flushing the caliper even disturbs the old fluid in the narrow annular space between the inner part of the piston and the bore. The inlet usually is near the bottom, but the outlet is always at the very top to bleed out the air. I am not even sure flushing the brakes carries any water or grit up and out of the caliper.
In time, the boot lets in enough to corrode the piston, both the plated steel ones and aluminum ones. Pushing the piston back with a lump of rust on it, either makes it stick, or the seal leak. Many times people will install new pads in calipers that were working fine, and then they either leak or stick. The solution is timely, preventative replacement of the old worn rubber parts and fresh lube on the piston. I prefer Sil-Glyde. A caliper kit is cheap, and rebuilding a caliper isn't that bad of a job. If you let it go too long, the piston is shot, and the cost of a piston and a kit is more than a rebuilt caliper. I have always rebuilt mine in time. After a rebuild or 2, all the original fluid is long gone before it can cause problems. On advantage of rebuilding my own calipers is that I know it goes back together with a good coat of Sil-Glyde on everything, especially the bleed screw, not brake fluid or the special brake greases that are just as hygroscopic.
Cleaning up the caliper and hardware, replacing the old, worn rubber parts, and lubing everything good prevents common problems like sticking or leaking calipers, uneven pad wear, uneven application, and broken bleed screws.
After 5 -10 years, it is good to replace the rubber brake hoses. Master cylinders are up away from the heat, salt, moisture, and grit, and seem to last much longer. If you have salt, it is good to inspect the steel brake lines paying particular attention to places where crud collects, holding moisture and salt against them. Badly pitted ones should be replaced.
In the 3 years I worked in a brake shop, leaking, sticking calipers, uneven wear, broken bleed screws, bad hoses, and brake lines rusted from the outside happened every day. We specialized in trucks. I never remember problems with brake fluid boiling. I never heard of replacing the fluid either.
