I've taken mine apart (00 750 ACE). Same petcock as yours. I used a couple of SHORT sheet metal screws to put it back together (and some Permatex #2). Inside these, is a ball and spring. If you blow into the outlet, it will let flow-by. Mine was full of "silt" and wouldn't function properly. I did get it serviceable again but wound up having to replace it eventually. They run just over $50.
Rusty ole bike rider dude ('84 VT-700, '00 750 ACE)