Well, eff. Took a closer look. The pistons are rusted to the calipers under the rubber seals, which weren't, apparently, sealing. They are not budging at all. I have them soaking w/ some penetrating oil but I fear the worst. So, how much are used ATE calipers like this (1989 911 Carrera, rears) and what is a good source?
ATE® - Porsche 911 Series 1989 New Brake Caliper
Or should I just bite the bullet and buy new (which I'd do from Pelican of course). Aggravating thing is, I'd need to have them powder coated to match the fronts, so that's more money.