Vacuum-operated petcocks are a handy thing to have on gravity-fed fuel systems; they automatically stop the flow of gas to the carb when the engine is turned off, saving you from a crankcase or floor full of gasoline should the float needle start to leak. But vacuum petcocks have their own problems. Over time, the vacuum diaphragm can fail. This will result in one of two things: A vacuum leak and a lean fuel/air mixture, or a gas leak through the vacuum vent hole. Neither one is really desirable.
The Big Cee petcock conversion kit will convert your automatic petcock to a fully manual one by replacing the diaphragm and rear cover. It includes the cover, nitrile (Buna-N) gasket, screws, and a cap for the carb vacuum port (or the vacuum tee on California models). Converting your petcock to manual operation has other benefits: You will no longer need to supply vacuum to the vacuum port to drain the tank, to fill the fuel bottle for your camping stove, or to loan gas to your buddy on the XR with the small tank.
If you install the kit on an older bike and find that gas still drips out of the petcock in the "off" position, your selector gasket is probably worn out. This is Kawasaki part number 11009-1188 (fuel tap gasket, check buykawasaki.com to make sure the part number is the same for your year); I don't carry them, but you can get one from your local dealer, or Arrowhead or Ron Ayers.
Part# PC, petcock conversion kit