What is happening that makes you say the air gap is gone? Are you equating a dripping or leaking pressure relief valve to mean the gap is gone? If so my bet is on either a piece of something holding the valve slightly open, or a failing valve. I don't think a leaky valve can deplete the gap as the valve is lower than the bubble(check Chris Bryant's post above).
My relief valve weeps from time to time, typically I reseat it and the problem goes away. Try letting the tank cool and then open and shut the valve a few times. I guess the valve is replaceable as a last resort. But I would be concerned that the aluminum threads would come out with the valve.
