I could see that being an issue with the pump sucking up air bubbles, and shooting them into the display. There's really not a reason to just have one line with the ball valve on it (unless you plan to feed something with the pump). In theory you could completely restrict the flow of the pump, and it would be fine. (as long as it's on the return side of the pump. Never restrict the intake of a pump, it will cause cavitation, and burn the pump up. But restricting the return side of any pump, is completely fine).