If you own a vintage steel frame, chances are the rear brake cable is routed through braze-on cable guides along the top tube.
Every time the rear brake is applied the cable housing moves slightly. If it drags across the top tube, or touches the seat-stay caps, eventually it will wear through the paint.
To avoid this, route the cable so it is slightly above the seat lug, clear of the paint, and the cable housing rests against the aluminum seat post, as shown in the top picture.
Try not to have too big of a loop in the cable housing, or it will push the side pull brake off center.
To hold the cable housing in this position, place a small rubber “O” ring just behind the last cable guide.
Cut a groove in the plastic sheathing of the cable housing, with a sharp knife, so the “O” ring will drop in this groove and stay in place. See the close up detail shot above.
Use a # 60 “O” ring (¼” O.D. x 1/8” I.D.) These can be found in the plumbing section of your local hardware store.