Your Vessel needs recharging with compressed air to 1-00 Bar, keep a drain valve open whilst pumping up to allow the water to discharge from it, as long as the air charge holds at 1-00 bar, the vessel can be deemed as OK, (it should be recharged at every annual service)
The Lock-out problem is not connected with the expansion vessel, and should be investigated by an oil engineer, it does sound like this boiler has been poorly maintained.
www.elanar.co.uk, October 2009
