Air bubbles in your returns ALWAYS, ALWAYS means a suction leak. Unless your pool has piping several feet in elevation ABOVE the pool, and large return lines, a suction leak is ALWAYS on the suction side of the pump.
Silicone caulk, not teflon tape, is the 'down and dirty' quick fix in many cases. But often the problem is the pump strainer lid gasket.
Ben
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