If u have problems with low pressure are u aware that u have to check/refill the boiler every 3 months with the filling loop. However if u are convinced that the expansion vessel is at fault then the simple way is to refill it with air. Any car foot pump will do. U will require to know how much u r putting in, therefore a pressure gauge is required. So heres what to do. Lower (or drain the boiler only) down to zero, The expansion vessel in most cases is located on the back of the boiler. its normally RED in colour (some later versions are in grey) fit the car foot pump valve to the valve on the expansion vessel. with the boiler drain off still open, re inflate the expansion vessel up to about 1 bar, maintain the air pressure until stable at 1 bar, remove the car foot pump, put back the dust cover, close the boiler drain off valve, refill the boiler to at least 1 to 1.5 bar, don't forget that there is now AIR in the central heating side and when this exits via the auto air vent, the pressure will drop.
the boiler technician