You shouldn't get an air pocket just dropping the level and then topping it off. Mine has all kinds of issues...heat...no heat...gauge spikes to hot and stays that way for a min ute....then a blast of heat and gauge goes to normal. Mine is a head gasket issue that I am just milking along. I have sealer in it and once it does "it's thing" ...it settles on normal and my heat is good until I shut it off and restart it...then it starts over again. As long as I keep the level about 3" lower than the lowest mark on the tank, it doesn't puke any out as it decides what to do.