Oem Gts Kelsey Hayes 4 Piston Calipers
I put rebuilt calipers on my car about 20 years ago. I just flush them out every two years or so and they still work perfectly. And they are not stainless steel.
Flushing out the brake fluid works beautifully, keeping water out of the system
Brake fluid is hydroscopic: brake fluid absorbs moisture
Piston bores pit due to humidity, but can be helped by driving the car regularly, where the pistons have to travel through the range of the bore.
Some people have found running silicone brake fluid in older cars, where anti-lock brakes are not a consideration, is also a very good alternative, as silicone fluid does not absorb moisture.
Driving these cars every month is preferred, but sometimes this is not possible due to bad weather (winter).
Changing the brake fluid regularly, though, is a good insurance policy against rust in the braking system.