Pulling the gas tank on my 53' 1/2 ton, and I came across what appears to be a fuel petcock valve. All of the diagrams I have seen for the 47-53 pickups have the fuel line connecting to a rubber hose that enters the passenger compartment from the underside. In the pictures it looks like this line connects to a steel line which enters that tank at the top. However the valve connects to the fuel tank at the bottom. I have included a picture, please note that 1/2 inch bolt at the bottom is stripped and the lever broke off when I attempted to turn it. Thought I'd ask about it before I remove the whole thing, and bypass it.
http://www.flickr.com/photos/kaliforniaman/4153731374/
