Water in the cooling system, is under pressure, Its temperature will be able to go above boiling point, without boiling. (Boyles Law) So undoing a valve, should release the excess pressure, That is basically what radiator caps did. As no water flowed out of valve, until you opened the cap, NO pressure in the system, So probably not enough water in the system. Opening the cap, allowed air to go in, so water came out, releasing the vacuum, rather than from a pressure build up. You might have topped the tank up, But what about the rest of the cooling system. When you start engine, at a certain point water will be pumped around. At that point, extra water will need to be added, to top up again, the rest went into the block, You will need to do that a few times until the block is full, and the top tank is upto mark. Then when opening the valve, without opening the cap, water will flow. BUT do not forget to close all valves etc, and have the water up to mark. YOU will also need to do something about antifreeze/summer coolant. AS none will be in your system.
SD
June 2012