Sheldon (about half way down)
Spring Adjustment
"If your wheels are centered, and your brakes are not, and, if the pivots are properly lubricated and free-moving, the brake shoes should be centered. If they are not, you probably need to adjust the spring tension on one or both of the cantilevers.
"Some brake bosses have
more than one hole for the end of the spring, to allow a rough adjustment of spring tension. Generally, the hole which provides more tension should be used with a narrower rim. Make sure that both brake arms use matching holes. [Edit: Comment: My experience is that there are other aspects in play so the 'matching' thing is a start point: the end point is getting each brake pad an equidistance from its rim and enough spring tension to move them away when the tension on the cable is released.]
"Most direct-pull cantilevers have a fine adjustment for spring tension. The adjustment will be a small screw [
@figbat ] with the head facing outward to the side of the bike. The screw is generally located near the bottom of the cantilever, below the pivot point. Sometimes it works with a Phillips screwdriver; other times, a small Allen wrench may be needed."