Air is most likely getting in from the waste port of the multiport, or the air relief on top of the filter. If it's from the waste port, then disassembling the multiport to check the gasket might help. When reassembling the multiport, lubricate the o-rings to help the multiport seal better.
You could plug the waste port until it's needed for backwash. Adding a check valve between the pump and filter can also be helpful.