Hi all! The filter mode on my heyward sand filter isn’t working. The pressure reads zero and the pump doesn’t fill. I heard a hiss from the filter, like air is escaping, but I can’t tell where it comes from. It works fine in backwash and recirculate., and when I backwash the water is clear. It’s been 3 years since I got the filter and haven’t changed the sand. Any thoughts as to what the issue is?