Be sure the spring anchors and spring ends where they touch are clean and free of rust and dirt so they slide smoothly and evenly.

Park, and others, sold small thin "cone wrenches" with the opening (mine are 10 mm and 13 mm) at 90 degrees to the handle so they could slip over the thin wrench flats on the nut behind the brake arms. You used it to hold the caliper centered while you tightened the mounting nut.