Float levels could wrong and set too high, but that won't cause a vacuum at the petcock when the engine is off. The petcock will only leak when the engine is off if it is defective. Unless you have it set to prime of course.
First, I fix the petcock/vacuum valve. Easy enough to check once its fixed or even before fixing it, by setting the tank having some fuel on a bench and the petcock set to off and no vacuum. So nothing should flow/drip out of the fuel delivery line. Not a bad idea to clean out the tank while you are at it. Install an inline filter. Very hard to get the tank completely clean. And you don't want to have to go through this again even if it is fixed this time around. The next step may not be applicable to your bike. Don't know if your needles are rubber tipped or not. If not, take your valve seat and needle and try lapping them. I used rubbing compound on the valve seat with a Que tip. And then some more compound and cleaned the needle tip. Then with more compound I lapped the valve seat with the valve needle for a bit. Cleaned off every thing with carb cleaner. This finally fixed my leaking issue.
P.S. I had originally replaced my needles and valves with some from the K&N rebuild kits I used to rebuild the carbs. But those seemed to leak. So that is when I put the originals back in after lapping them.
Here is a link of the
carb rack holder
I constructed which works with a workmate bench. Helps make leveling the floats easier. I should have made it so it had legs on the bottom and top as well so I can turn the assembly upside down when needed to make it easier to work on once I determine a float needs adjustment.
