You will need to set the expansion vessel to about 1.5 bar. You should have it so that it equals out the pressure in the boiler but trying to this is tricky so I usally set it at 1.5. The system should have no pressure when you pressurize the vessel. The problem could be that the vessel is Knackered. It has a rubber diaphragm on the inside of it that can fail. You can test this by depressurizing the vessel while the boiler is under pressure. If you get water out of the schrader valve then the vessel is done. Before you try this though tap on the vent with the handle of a screw driver to see if it resets itself.