If you are getting air into the filter, i.e. you have to open the vent valve on top, then you have a suction- side leak.

Check the O-ring to the pump basket has Magic Lube, or similar.

Check with a 3 - 4' piece of PVC pipe held against one ear, the other at different places on the suction piping. When you hear bubbles, that's where the leak is.

Hope this helps.

Pat