I have a friend with an above ground pool who is suddenly having a problem and asked advise. His pump basket now will not fill up completely. He has a regular pump and a sand filter. The pump basket will suck in some water, push it to the filter, but never full fill with water. He's tried opening the filter to make sure it is full of water and opening the skimmer basket to do the same. But he keeps getting air into the basket and the pump cycles water then air. Any suggestions or thoughts?