I keep getting air in my boiler water pipes what could be wrong

My boiler makes a humming sound at the end of its pumping cycle. Normally I would just purge the water pipes and some air would come out and the noise would go away. Now when I purge it the air bubbles keep coming out even after 5-10minutes. I recently had a white rogers zone a flow water value replaced. The boiler is a Lochivar.

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