Pool was started up yesterday...went out this am to clean skimmers and backwash (filter was showing 40 psi ). When I turned filter to backwash position (sand filter, Pentair TR100), I heard a lot of gurgling and pump basket drained of water (Pentair Whisperflo). I then shut off pool valves and went to fill pump basket with water...it reached a level of about 3/4 way up and then just maintained that level despite me continuing to fill with water. Of course, pool company is closed today. Am I doing something wrong/missing something or does it sound like there's a leak somewhere in the equipment area? Thanks for any and all advice!