Sidepull brakes often use a cone wrench to center them. If there are flats on the side of the bracket that holds the spring, that is what it is for. It sounds like you need new washers between the brake arms and the bracket. Or try to disassemble the brake and grease those areas up. If you or somebody else have already reassembled the brake you may have washers misplaced. You may also have the brake tightened up so that the additional pressure from the rear nut causes extra friction.