My 61 had this with the left rear brake. It turned out that the newish brake cylinder was rusted inside and therefore the cups did not re track as fast as the other 3 cylinders, causing this one to drag. Dismantled the cylinder and with very fine sanding paper and brake oil cleaned it up. Now perfect. If the truck is not driven a lot like mine, this seems to be a problem. So...the solution is to drive it more!
