Hy Pipeliner78, Unfotunately the middle to late 60s are a time of change for truck braking systems. Is your master cylinder a single or dual system type? The rear brake shoes appear to be the same from 1966 - 1970 on the 15000 - 28000lb. two speed axles o.e.m. p/n 3912439. Your rear brake system could be General Motors automatic adjusting type, Wagner manual adjusting type, or Wagner automatic adjusting type, I'll try and narrow it down a bit more. Any chance of posting a picture of what you have left?
