Air in the system means a SUCTION side leak -- that's everything between the pool and your pump's strainer basket lit. Leaks past that point let pool water OUT, rather than drawing air IN.
Probably the two most common places for suction leaks are
1. where the pipe from the skimmer &/or main drain enters the pump, &
2. around the pump lid gasket.
You can check the inlet threads with silicone: if you wipe it around the threads, and a tiny bit gets sucked in (or, if you're lucky, the leaking stops) then you've found the problem. Also check the pump basket gasket -- remove any dirt, replace the gasket if it's warped or cracked or whatever.
After those things, you can check valve seals and unions on the SUCTION pipling.
Good luck - finding small suction leaks can be a real pain in the @##
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