New T's as well. If they filled ok, they should feed ok. When defueling, do they go even? Did you replace with all new line. Could have a small pinhole.
I'm betting on a pinhole in the vent line on the tank that's remaining full. Therefore, not getting pressure to that tank causing to pull on the one tank. They would fuel and defuel fine. Try pressurizing that side by itself, check all the vent connections.