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If there is no air pressure in the tank. use an air gauge you should have 12 psi. But remember you let air out every time you test it. If it does not have 12 psi you can pump it up to 12 psi with a pump. But this is just a temporary patch and it will go flat again. so it should be replaced.
Although a water logged Expansion tank will cause a relief valve on a boiler to leak and /or blow off. It is not the only reason the relief valve leaks and or blows off.
See why is my relief valve leaking? by clicking here

Expansion tank