You only need PRI to fill the float bowl, which takes only a second or two. Then switch to ON, at which point gas will only flow when the bike is running, creating vacuum to open the diaphragm in the petcock.

You should have just dropped the bowl, then turn to prime and gently move the float up,...which should stop gas flow.
Don't tighten the bowl screws very tite when you put it back on,....and untill you're sure you found the problem.
Float sticking is never a problem, unless assembled/adjusted wrong, or dirt in the fuel.

It doesn't take much at all to cause a leak at the needle and seat,I worked on a friends old 70s dodge that had a carb that would flood gas like crazy making the engine impossible to start.when I tore it down I found what was literally a speck of dirt on the needle.cleaned and reassembled and the car ran like a champ,no more flooding causing hard starts too.one tip I can offer is that when you get the bowl off,if all looks good as far as assembly goes,you can test the seal by attaching a hose to the fuel inlet on the carb and blowing through it while lifting the floats.if its working right you shouldn't be able to blow through it while holding the floats up,if you hear it leaking then you'll have to tear it down and see what's going on.good luck.