I will do it tonight when I get home, the heating is on continuos since yesterday morning so the pressure will be around 2 bar on the boiler, I will test it see what it says, thanksI think it is a top cap on that one. If so it could be almost full of water!
Did you measure the air pressure as you increased the water pressure and see what change occurred?
Tony
Valve cap is on top of the vessel it's like the back bone of the boiler,