I can't see a way to test that other than removing the tank, empty it and pressurise it through the bottom outlet port, then the valve should open at 1.4 bar ... be aware that the cap must be tightened to maximum possible otherwise the bottom oring is blocking the overflow hole on the tank where the excess of pressure should leave the system
if you presurised the system through that yellow hose the valve doesnt work cos the pressure goes through it