To drain the tank via the petcock you will have to apply a vacuum to the port on the back of the petcock.
A bit of hose stuck in your mouth and sucked on should do it, but it will get tiresome if you have a full tank... ;^)
There will be a fair amount of fuel stuck on the right side of the tank, too, that doesn't drain to the left side. If you want to get that out you'll either have to remove the tank and slosh it over or lay the bike on the ground to get the fuel to the left side. (Remember that if you ever 'run out of gas')
Get a syringe from the pharmacy. They are handy for suctioning out the last of the dirty fluid in master cylinders and all sorts of stuff. Use the syringe in the vacuum hose to pull a vacuum on the petcock and can walk away while the fuel flows. I sometimes use this technique for filling a carb before starting or draining fuel through the carb.
And, for just an ordinary fuel tank draining . . . be sure to put the petcock lever in the RESERVE position, to drain the lower reaches of the left side of the tank (some residual fuel will remain on the left side, as mentioned above.
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