Chicago or Don Mainland frames?
Whatever was in the backlog, Schwinn didn't do any warrenty work for several years through the early eighties. I worked at the shop that cleared all the warrenty backlog while Schwinn moved production the Waterford.
Most 70's era frames have their bad examples, Masi, Colango, Bob Jackson, Raleigh, etc. I remember when Raleigh released 753 tubing and wouldn't sell it until they trained the builder in a brand new brazing technique...silver
It's very difficult to have complete brazing penetration when brass is used, esp in the BB.