I believe I have an air leak in my sand filter, but I'm not sure. Here are the symptoms.
When the pump first turns on, it takes 15-20s for it to get fully primed and the pump basket completely full(it works fine after that). When the pump shuts off, I can hear water cascading in the filter, and, about 10 seconds later, the top of the pump basket gets filled back up with air down to the level of the incoming pipe. I read that the pump basket should stay full of water after the pump shuts off, is that correct? I replaced the O-Ring on the pump basket lid since it was showing signs of age, but it made no difference in the behavior.
A secondary symptom is small air bubbles in the returns. If I backwash the system, even briefly, the bubble will go away for a few days, but then start to reappear.