The main problem I found is just the rust on the actual pad carrier. When they rust, overtime, the build up will compress the little metal slider clips which the pads sit in, when that happens the pad won't slide back. I took a dremel to mine and removed the rust, greased them up and that should stop them seizing. A proper rebuild of the actual caliper is the next step, as above if the seals for the sliders have torn they may be seized too, a clean with some scotchbrite and then a grease up (grease is supplied with the toyota rebuild kits by the way) and they should be all good. You can see one of the sliders in the right of this photoae; The fact it has turned purple shows its seen some heat!