[jlex]

One of the most telling ways to determine if you have a dragging caliper is to note how it rolls at slow speed on a level surface. If the car stops short of a normal coast, start looking for a bad caliper by jacking the car up and spinning each wheel.
It's the internal seal which causes the piston to retract into the bore. If the seal is compromised, it doesn't take too much corrosion to disallow the seal to do its work.