06 f350 122k miles dually. Rear brakes were dragging noticibly after new rear pads put on. Was setting for 18 months when i bought it. I just put new rear lines on and still is dragging. New pads put on a week ago. If i break the bleeder free they dont drag until using the brakes again when it builds pressure. Could the problem be an internal caliper issue or is there something else going on?