How long ago were the rear brakes "fully" overhauled?
Reason for asking:
1. Because of having "old" grease, the petroleum liquid material in the grease leaks out & evaporates, thus leaving a "dry", hard caked residue all around the brake adjusting wedge "and" the brake adjusting shafts.
2. This dry caked residue fills the threads & makes the brake adjusting wedge not only harder to turn, but can pile up & jam the threads such that the brake adjusting wedge will not turn even with brute force.
3. If so, one gets a false impression that the brakes are fully retracted; but they are not.
4. If the adjustig wedge can be rotated, maybe try injecting Kroil or Liquid Wrench plus WD-40 into the back side of the backing plate at the rotating square lug; then "turn both ways & inject Kroil" repeatedly, several times, to try to soften this dry caked old grease.
5. After a while, you may be able to turn the wedge a few more revolutions in the counter-clockwise direction to fully retract the brake shoes.
Worth a try prior to wrapping the brake drums with TNT & a blasting cap.