We have an old (25+ years) Sterling three valve shower/tub assembly. Valves for hot, cold and diverter.
The diverter valve works ok for 100% tub, valve closed position. The valve open position, delivers water to both the shower and the tub spout. (Open/closed functions seem reversed to me, but that is what they do.)
I replaced the washer on the diverter valve stem. No help.
I cannot get at the assembly without destroying the tile wall.
I don’t know if I could remove the tub spout without damage. Getting the valves off was difficult – I ended up destroying the trim because all the threads are frozen or corroded.
If the closed position was working then the washer was okay. There is another washer, ring gasket, or something above the washer that seals when the valve is in the “open” position. That is the one that needs replacement.
When the valve is open (flow is intended for shower)hot water only flows to the shower. The tub spout gets all the cold.
There is a place on the valve stem that seemed to me to be a place for an o-ring. But there was no residue or any other indication that anthing had ever been there. I asked at the store, and the attendant there said no. I will return and see if I can find an o-ring that will fit.
I looked into the wall with a flashlight, but I am still unsure that I understand how the thing actually functions. In some other post here or at another board, I saw a note (unrelated to my inquiry) that suggested favoring cold over hot would mean that there is a balancing feature, and to look for a screw-like device in the assembly. I looked, but did not see anything like that.