Do you have trouble in centering side-pull brakes? Here’s a simple little trick that I have always used. First, tighten the brake; don’t worry too much about centering the brake pads at this stage.
With a flat punch, (An old bolt or ¼ inch socket extension works well.) and a small hammer, tap on the top of the brake spring as shown below. Tap on the right or left side, moving the pads in the direction they need to go to center.
What you are doing, is not bending the spring, a light tap with a hammer will not do any harm. By tapping on the spring you nudge the brake bolt into the center position without loosening it.
Make sure your brake cable housing is not too long, or too short, or it will constantly push or pull you brake pads off center.