The overflow is running because the boiler pressure is too high because there is too much water in there...if the expansion vessel does not contain any air then this indeed needs fixing. If water squirts out when the air valve is depressed then it is goosed. Price and replacement depends on the boiler, vessels are up to £100. Personally I would want to see the PRV (pressure release valve) holding back the water first - they don't let go until 3 bar.
John, October 2009
