Purging the HW tank to get sufficient water to protect the element is one thing. That is adding enough water to the HW tank to to submerge the element and keep it from burning up. You need to let the water ( including cold) run a while to get all the air out of the lines to keep the pump happy. That includes the cold water. The other possibility is that the pump is drawing in air with the water due to a loose fitting. The air will compress and the pump will run. Try purging the hot and cold water at the faucet the longest distance from the pump.
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