That's still vague. How did you determine that it's stuck? Where exactly are you seeing something not moving that should be? Did you take the caliper off the rotor & watch it while someone pressed the pedal?Sorry I was vague, but its a stock ford disc brake caliper piston that is stuck.
Phenolic or steel caliper pistons? While phenolic don't rust, I've seen them do some other funky stuff inside calipers.






