I just happened across this site by accident. I'm no expert, but usually hot water expansion tanks have an internal bladder. Usually air on one side and water on the other. If you get water out of the schrader valve then the bladder is bad and a new expansion tank is necessary. If the expansion tank bladder is bad the tank will be full of water and the system has no where to relief pressure except the relief valves. The relief valve in this case will relief when, say, a dishwasher or close washer is being used. Any appliance that uses water. When the dish washer is full of water and a valve shuts the water off suddenly it creates a hammer and the bad expansion tank can't take up the sudden added pressure so, the relief valve "pops off"
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