On a suggestion, I tried using the lower speed of my 2 speed pump. It still has some suction through the hose and vacuum head to pick up leaves although not as well as on high.
The filter pressure gauge shows nearly zero pressure. The pump trap does not stay filled with water on the low speed, with water just gushing from the inlet to the pump impeller opening. There is no obvious air bubble. Is this enough to effectively pump water through the filter?