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
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