So what prevents fuel from leaking out of those two infamous vents on top of the fuel tank when it is filled to the filler neck? I'm thinking of plumbing my in bed auxiliary fuel tank to one or both of them. Thanks!

 

They are equiped with rollover valves. So they will close themselves off if fuel rises up to a certain level or if the truck rolls over.

 

Thanks Spooler, but how does a rollover valve work? A float valve cannot do both of the things you mentioned.

 

Usually a roll over valve consists of a tube that has openings and a ball that will float in fuel, if the fuel level rises the ball will float and seat on the top of the tube preventing fuel from coming out, If the vehicle rolls the fuel trying to rush out will cause the ball to seat also as the fuel pressure trying to come out is greater than the floating tendency of the ball.

I haven't opened up one from our trucks but that is typically how they work.