I agree with Tester about the rubber hoses. These are the most common cause of dragging brakes I have ever seen. Many people, including mechanics, needlessly replace calipers in a fruitless attempt to cure a dragging brake problem. Another cause other than what Tester mentioned is the steel bracket that mounts some of these hoses, which wraps around the hose. Those steel brackets sometimes rust, which constricts the hose and causes dragging brakes.