Either your system has been over-pressurized
if so, make sure your filling loop is not turned on, the bleed the pressure from a vent in one of your rads until you get back to about 1 - 1.5 bar pressure.
If the problem persisits, it could be your pressure vessel is fecked, open the valve on the top of it, press the centre (like your car tyre valve) for a second, if you here air it maybe ok, if there is any sign of water...fecked! and call a service engineer to replace it.
Hope some of that helps
Tony, February 2008