Odd thing , just the slightest air in it now ( running the last 2 hours ) , earlier today went to 1/2 empty and I re-primed after only an hour. The only difference is this time I didn't bleed the air from the filter ( which it is my understanding I'm supposed to do ) . Might be a coincidence, just thought I'd mention it.
This could indicate the air is coming from some place else like a suction cleaner rising above the water level or a skimmer weir door that is stuck so the skimmer is sucking air.