That's interesting that the pressure only went up to 20 psi. I'd double check two more things before deciding to cap it off:
- Are there any plugged ports anywhere in your pool? It could be a pressure side cleaner line.
- Try what you did with the valve again, but this time close off the main drain line on the suction side of your pump. It's possible the mystery T returns to the main drain, but with that line open for suction you could get bad hydraulics with no pressure build up, a lot like you saw.
And check around the yard for a capped 3" stub anywhere. Could have run a fountain or something at one point that was later removed.
- Are there any plugged ports anywhere in your pool? It could be a pressure side cleaner line.
- Try what you did with the valve again, but this time close off the main drain line on the suction side of your pump. It's possible the mystery T returns to the main drain, but with that line open for suction you could get bad hydraulics with no pressure build up, a lot like you saw.
And check around the yard for a capped 3" stub anywhere. Could have run a fountain or something at one point that was later removed.