No problem, if you can find the expansion vessel (usually a red tank) and also the car type air valve near the top.....check its pressure with a car tyre gauge - if water spurts out though, go no further but replace it. As you pump air in (using a bike or car tyre pump or similar) it will displace some water, and to get an accurate pressure reading this volume of water has to be bled off at a radiator, or at the pipe leading from the expansion vessel - if its visible. Anyway, the vessel needs an air pressure of 10 psi to work properly, and thats how much air you pump in. Its my guess its rather empty of air!
John
October 2009