Assuming there's nothing wrong with the cables and the calipers, upgrading to Kool Stop salmon pads (same pad formula as the old Scott-Matthausers) and aero brake levers is the way to go for noticeably better braking performance. On my '83 Schwinn le tour luxe, I've got Shimano Exage aero levers ($15 at my LBS), the original "Schwinn Approved" centerpull brakes, and cartridge-style V-brake pads (w/Kool Stop salmon inserts). This setup works so well it amazes me-