heeelllooo and welcome pantiome

turning it to zero should push the pin in the valve shutting off the flow
no need to drain
i personally would turn to 6/ max unscrew the head from the brass body and "play" with it and see what's sticking once you are happy its free oil the pin on top off the brass leave for 5 mins then wipe clean
then make shure the valve is set to max/6 to make it easier to fit making shure you dont cross thread as you refit