Several things need to be addressed. First, find a nut to put on the pivot bolt (in addition to that already there). Then, adjust the pivot so that it moves easily without back and forth play. Lock the two nuts together they same as you would a cone and locknut. Next put an adjustable crescent wrench on the part of the caliper arms that hold the brake shoe and bend the brake arm so that the front of the shoe barely touches the rim first (toe-in). Now you can adjust the brake normally. If it's off center try tapping the spring where it loops on the side that is out too far, to rotate the caliper slightly.