Bubbles in return when using sand filter

[Granny]

Yesterday I noticed bubble coming from my return line. My pump appeared to have full head, there was plenty of water in the pool, but the sand filter was making a "gurgling" sound. When I switched to recirculate instead of to filter the bubbles stopped within just a few seconds. What's up with my sand filter?
I appreciate your help,
Dee

 

[1Sammy]

Air getting in and no water getting out has to be on the suction side. I would take a closer look in the basket to see bubbles. At times they can sneak in and stay close to the edges so hard to see. When you bypass the filter you increase the "pull" on the suction side and that causes the air gap to suck closed, that why it stops.

 

[Granny]

I am seeing absolutely no air bubbles in the basket. None at all, not even at the edges. I'll see what I can learn about problems with the suction side. Maybe try putting bypassing the skimmer while using the filter, and if I see bubbles bypass the drain while using the filter.
Thanks for your help,
Dee