I put silicone lube on the oring and tightened it down. The sensor was not reinstalled when I redid the pvc from the discharge to the filter (pump position had shifted due to the unions). No sign of any leaks anywhere. There is a 3 way port with valve actuator on the line coming into the pump. Wondering if maybe there is a leak there where air is getting in. I would think I would see water leaking if that were the case though.