I hate to the the bearer of bad news, but I would take a hard look at the entire brake system including: master cylinder, lines, fittings, rubber hoses, wheel cylinders, shoes, drums, etc.
While you may not have to replace the entire system, be prepared to. On these old systems, when you "fix" one component another problem becomes more evident. Walcks sell a great overhaul kit that includes everything except the steel lines....which they sell as a seperate kit. Total cost is around $800-900 depending on sale prices at the time of order. I used them and the quality and fit was great.
Good luck and don't skimp on safety!