how do i check a valliant combi boiler expansion tank?

you depress the car valve on the expansion vessel, no water should come out if it does diaphram gone a new expansion vessel needed.
if just AIR comes out you will then need to re inflate the pressure vessel back up to 1 bar max with a CAR PUMP with NO pressure in the waterside of the system while you inflate the pressure vessel .

wirecutterman
November 2010