After many hours I finally discovered that as my Martin D-28 is a pre-1985 guitar (1980, specifically), it has no adjustable truss rod. I did everything, I even dropped my cell phone down inside with a flashlight to take video. It was like looking at video of the Titanic or some sort of verite horror movie. Anyway, no available truss rod.
That is a video demonstrating the lack of available truss rod. Could be from a horror movie, I don't know.
Warren Ellison is a luthier in Vermont.