air bubbles returning to pool

[manio]

My skimmer has two holes at the bottom of it. One is connected to the main drain and the other is the suction to the pump. No air bubbles when I operate the equipment in this manner. I have however constructed my own piece of plumbing to isolate the suction from the main drain hole. Simply put, both holes are covered with a u-shaped piece of connected fittings to draw water from main drain. It is when I have this connection when air bubbles return to pool. Why?