This is intended to be done with the carbs OFF the Bike. and for needle valves that DO NOT have rubber tips. When ALL else checks ok, and ONLY AFTER, your carbs STILL leak gasoline out the overflow hoses,
and you're SURE its due to the float valve not sealing properly, the valves and their seats CAN sometimes, be cleaned successfully. As long as the float valves and float have no other level/ wear/sticking/alignment/dirt/corrosion issues, try polishing the valve tip and it's seat with 0000 steel wool. Polish (by hand) the valve tip (aluminum tip only) then, polish the seat by hand with the steel wool formed into a small "cone" wrapped around the end of a small piece of wooden dowel rod. Assemble the floats and turn the carbs so that the floats close the valves, then try to manually blow through the fuel inlet hose by mouth (use a clean hose if you wish) which properly sealing valves should prevent you from doing. I found this to work very well. Re install the bowls. Do a "wet test".
Get a pan, set a pair or rack of carbs in the pan and prop them upright. Run a hose from the bikes fuel petcock to the carbs, turn on the fuel and look for leaks. You can do the clear tube method at the same time if you want, a good idea if you are checking 550 carbs and don't want to remount them till you know they're right.
There are MANY different ways to perform this operation, this is just a suggestion, that I found to be the easiest and most thorough, having just done it on a 550K,
and others prior.
THE INFORMATION CONCERNING THE USE OF 0000 STEEL WOOL AS A POLISHING
METHOD WAS OBTAINED FROM THE TECHNICAL/FAQ WEB PAGE OF CYCLE-X'S WEB SITE where credit is due.
