We have a pentair variable speed pump. 38k gallons. Sand filter with multiport valve. I am able to prime my pump. However, if I switch to backwash on the multiport valve it looses prime so I can’t back wash. My pool store recommend that i prime the pump in backwash mode, I am questioning if this is safe to do. She thinks that the reason I am losing prime is that my sand filter is too dirty? I have read nothing about that being an issue. Do you have any thoughts on this?