I had the same problem, but with the rotors, not the calipers. The flange part of the rotor (the part that the brake pads no NOT touch) were rusting. This part of the rotor was not painted very well, and caused them to rust. I had Ford replace the rotors. I know some folks will read this and think "painted rotors" thats crazy. But, this is the flange part of the rotor, not the friction area.