I think the Extrol tank has a schrader valve for control of a precharge pressure. Have you removed the pressure *totally* and then measured the precharge pressure? I don't know what the pressure should be for you, but I am thinking it should be set to whatever pressure would cause water to be added to the system. I think the initial precharge is 12 psi, but you can change it with a tire pump to fit your system.
If water comes out, the expansion tank needs to be replaced.