Can't Stop 71 440 Demon

I ordered pistons and boots/seal kit but the caliper itself is rusted keeping the seals from sealing.
Get yourself a couple of gallons of Evaporust, and soak out the calipers, to bare metal condition.

If you don't own a Dremel, go get one. Buy, beg, borrow.
Put a flap wheel on the end of the Dremel, and then use it around the bore of the caliper.
Cleans them up nicely.
Don't loose any sleep, worrying about, doing it.

I see your Michigan, a rust bucket state, so maybe your calipers are pretty rusted.
But i never have seen any Kelsey Hayes calipers that i couldn't rebuild, except for rusted, frozen, parts.

Dremel flap wheel.png