Seems like I have the same issue every year. I blow the lines from the pump. Plugging the suction side (gizmos and main drain) all go well. Then the returns start blowing. I use the hayward threaded plugs with o-ring (and teflon tape) to plug the lines. The returns are below water. While the Mighty Vac is blowing, i always see tiny air bubbles after plugging each return but they go away as soon as I turn the blower off. This bothers me, but when I called the pool company that used to do the closings, they say not to worry about it as long as when I turn off the blower there are no air bubbles.
Should I trust their response? Like I said, this happens every year I close, and I never have any issues, but I'm wondering why those plugs do not seal air-tight. Anyone else see this?