Air in the strainer is caused by a suction side leak. The most common is the Oring on the lid. It needs to be clean, lubricated, and free of cracks. Next most popular is low water level and/or a stuck weir door (skimmer flapper). If you try the search box for suction air leak, you'll find more than you ever wanted to know.
A variable speed pump running on a low speed will sometimes do this even without an air leak. If there isn't an air leak, you won't lose prime. If you are losing prime, or running a single speed, or two speed pump, or running a variable speed pump at half speed or higher, there must be an air leak.
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