Plumbing – Who manufactured this shower diverter valve

The plumber came out today and showed me how to remove it. The Evil Greebo was right, the stem itself unscrewed. The plumber took off the handle and escutcheon, then put the handle knob back on to further unscrew the stem. He did this very slowly and used a lot of force. He had a lot of difficulty with the cold, and that is the one I had started with, so it is not surprising I was doubtful of this being the way it was supposed to work.

The plumber thinks it is Sterling, because those were popular around here years ago and they were low cost, the type that someone would put in the downstairs shower.

Sure sounds like the diverter needs fixing or replacement.

When the diverter is closed (sending water to the showerhead), it has to handle the full supply pressure. When only closed two thirds, it sees significantly less pressure. Sounds like the force of the water, is pushing the diverter open. This allows water to flow out the tub spout, rather than being sent to the showerhead.

Depending on the make and model, you might be able to make adjustments, replace parts, or a full replacement may be required.