I wonder if the inner caliper piston(s) have partially seized. After cleaning rusty inner disc face, and if you feel confident, removing the inner pads or maybe both then inserting a thinner piece of wood (each side of necessary) should allow you to operate the foot pedal without the pistons popping out. Once they are gripping the wood, carefully push them back in using the wood as a lever. Maybe repeat a few times until they all move freely then re-fit pads with Ceratec brake grease.
