Had this issue with a few Yamaha's in the past.......but fortunately NOT with my 550......although due to lack of everyday use, I inadvertantly snapped off the lever right @ the pivot ....which coulda been cured with a shot of WD.
The gaskets in the tap assembly itself tend to dry out & become brittle after being stored for some time.
I ended up buying a new tap from MOTOGRID which was $35 well spent.
FYI....the MotoGrid site is great for referencing with the exploded parts views.