If valve doesn't seal when closed:-
With cold feed/combination valve open, turn off all hot water outlets and open valve again to briefly flush seating by discharge through tundish. May take 3 or 4 attempts before valve seats properly. Air pocket will be unaffected because cold water entering replaces discharge through PRV. If this fails,as above, G3 and new PRV.
With cold feed/combination valve open, turn off all hot water outlets and open valve again to briefly flush seating by discharge through tundish. May take 3 or 4 attempts before valve seats properly. Air pocket will be unaffected because cold water entering replaces discharge through PRV. If this fails,as above, G3 and new PRV.