Howdy, and welcome!
I had to pull my fronts off, pop out the pistons, and give 'em a healthy scrubbing. New seals from our host, a 10 minute bleed, and no more mushpedal. Mind you, this was after I realized I had them on upside down, and had been bleeding them for most of my adult life. You mention one of the rears is crusty and questionable (theres a bad joke in there somewhere..) - It's probably a good idea to pop the pistons out and put in new seals, that was really what my problem was after all was said and done.
Good luck, and good bleeding
Jeff