there is no cut off you have one main fuel line with a fuel strainer in the tank if that line is leaking you need to siphon all fuel out and get a new line you can stop the fuel leak with a pair of vise grips crimp the hose to stop the fuel from leaking, you have three lines one is a vent one is a return and the odd one by itself is the main fuel line, all these metal lines can be made up with new line 5/16 will work you can use pex also very cheap and works great will not rust just use hose and clamps to connect it where it needs to go. 5/16 pex......