It's just an 0-ring and seal inside.
Some say that if the piston is shot, don't bother trying to sand it clean...others say that's fine(ish).
You need to blow the piston out somehow...with compressed air, or before you remove it from car...remove pads, put some wood in caliper and press the pedal.
Remanned caliper is not expensive, and if piston is rusted, might be the way to go (pistons are expensive(ish))
No pics from me...sorry.