Air from main drain?

Does the main drain-piping run all the way along side the inground pool straight to the pump? I'm getting air bubbles in my pump basket when I have my Jandy valve straight ahead, when I close off the main drain the bubbles go away. I don't believe it's the suction problem at the pump because when I close that valve the bubbles goal away for the main drain. I tried running water around the pipes and it didn't seem to make a difference. I'm certainly hoping there's not a cracked pipe underground. I would appreciate any input or advice on this matter thanks Randy