[soundman98]

all calipers have rust within them like that. it's inherent to the design of all calipers. it's a cylinder of captured air-- moisture gets in there and will create some surface rust.

you could replace them, but the new one's will rust just the same, just as fast.

the only part to ever be concerned about is the part that the rubber boot covers. that's the part that seals in the brake fluid, and creates the hydralic pressure that makes for stopping. if that starts corroding under the boot, that ruins the seal, which leaks fluid, which can make for a very bad day.