Drum brakes will drag slightly when adjusted properly. By slightly, I do mean slightly. Even freshly turned drums are not perfectly round. They will touch the shoes at least on one spot when rotated.
The shoes should be adjusted and the brake pressed and released to center the shoes. Then spun and rechecked for contact and usually readjusted again.
It is important to re-center shoes after each adjustment. It is also important to check the contact pads on the brake plate. The pads must be clean, rust free and smooth.
Ridges worn into the contact pads will prevent the shoes from returning to center and prevent the proper adjustment.
Ridges are common on these old trucks. Usually it's easy to weld up and grind smooth.
