The petcock usually has a brass tube to feed the "ON" side that pulls fuel from a higher point inside the tank. If the brass tube falls out then both "ON" and "RESERVE" are pulling fuel from the bottom of the tank.
1999 750 ACE Deluxe (Shadow) petcock valve
I ran out of gas this morning on my way to work! A first for me in 11 years of riding the same bike. I knew it was getting low as it started it's usual sputtering, so I flipped it over to reserve, only to find that the tank was truly empty. Can the petcock valves on these bikes go bad after 30k miles and 24 years? (Had to walk almost 3 miles to work 
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