Mine will do this too, with or without the sock. Anytime there too much stuff in the skimmer it sucks in air from somewhere. My guess is somewhere else in the lines, or maybe the dome on top of my sand filter. It does the same thing if the filter basket in my pump is too full. I don't worry about mine since I'm not losing water, and the air showing up is an easy to spot sign that I need to spend a little time cleaning everything.
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