I had one that had a slight tear in the dust cover, some water got in there and rusted/scored the piston well. It wouldn´t retract all the way, unless I used a large C-clamp to compress the piston, with the bleeder valve open. I usually noticed most with new pads.
Make sure the piston goes in all the way, when installing new pads. If the piston is in all the way and the pads still stick/hang, they may be too thick. Sanding off .030-.060", isn´t going to hurt anything.
I had one that I refilled with contaminated DOT 3, when the brakes would get hot, it would make steam and lock down the pads on one side.
Normal caliper piston, will compress fairly easily with the bleeder open. I usually use a large set of channel lock pliers. If the pistons are sticking, it usually takes a C-clamp. A little piece of wood behind the caliper, will help finding/making a flat spot for the C-clamp.
When the piston is backing off properly, you can slightly move the pads, when the brakes are released and/or pry them slightly open with a small screw driver. If you apply the brakes and release and the rotor doesn´t turn without significant drag, the piston is probably sticking (a slight drag is normal).
A bent caliper rod is always a possibilty, I´ve got a bent one now, but it doesn´t seem to bother the braking much, but have noticed the pads wear slightly more on one side than the other.
caliper should, move/slide on the rods. Have seen the teflon insert get crossed up and cause the caliper to hang some on the rods. Then one pad catches more than the other.
