Typically, when your petcock goes bad one of a few things can happen. If the petcock has a diaphragm inside, and it goes bad, the petcock will leak externally. If the seals go bad inside the petcock, it will allow fuel to pass when the valve is closed and in some cases will slowly siphon the gas out of your tank. If the petcock fails due to debris or corrosion, it will simply not allow gas to flow.