Are you getting sleeved calipers?

When you replace them do this, get a turkey baster and suck out all the fluid from the master cylinder, wipe clean and replace fluid.
BEFORE you remove the old calipers, crack the bleeders one at a time and gravity bleed/flush (do not pump pedal) the caliper till the fluid runs clear, rinse/repeat at each wheel. Just keep pouring in new fluid into the master.
Check the condition of the flex hoses at each wheel, they can be problematic.

When you replace the calipers, mount them and hook the lines up and again just open the bleeder and gravity bleed (no pumping) until the fluid comes out solid and clear, no bubbles. One wheel at a time.

After that when you pump the pistons/pads out to the rotors, use slow short strokes until you get pedal.

You do not want to push the pistons in the master into the area where they do not usually travel, this will destroy the seals in the master.

Go back the each caliper after you have pumped them out and again GRAVITY bleed only until the fluid runs bubble free.

After that you should only need someone with a foot on the pedal for only an couple short bleeder cracks to push out remaining bubbles.

When applying pressure on the pedal make sure the lid is on the master (does not have to be latched down) or fluid can shoot up onto the paint. Brake fluis is excellent paint remover (I am sure you already know that).

After that every two years remove the fluid from the master cyl and refill and gravity bleed the system and you will never have corrosion in the brake system again.