If the tank is a proper heating expansion tank with a diaphragm, only gas should come out of the needle valve on the bottom of the tank (and it shouldn’t be fooled with or there will not be the proper pressure to do the job.) Water indicates that the diaphragm is cracked and the tank needs replacement.
If you are talking about the small canister on the top of the air scoop above the tank, that is an air vent and water will come out now that there is no air in the system. Much water removal this way will lower water pressure and permit air to enter the system. Read the air purge procedure at http://www.hydronic.net for a fuller description of what is happening.