The Polaris does have a booster, but I'm not sure how I would isolate that return. I do believe that it is getting better with the new sand, though I don't know why that would make a difference in that one return. I have attached a picture of the pump set up (oops my profile won't let me post an image). I'm just not sure where to check. There are two flexible hoses on that booster pump. They have never been replaced. Is it possible that there is a pin hole in one of them causing some air? The pump basket has air sometimes and sometimes it doesn't. Let me know if there is a way to post a JPG photo so you can see how it is plumbed. Thanks for any thoughts.