Gas vent tube?

[71CHEVYSHORTBED402]

Originally Posted by ray_mcavoy

That vent line (that currently ends off under the floor) originally ran up to a charcoal vapor recovery canister in the engine compartment. The canister stores up gas fumes that evaporate from the tank. A purge line ran from the canister to the carburetor to draw out the fumes (and burn them in the engine) when running.

The loop that runs up inside the cab is a liquid-vapor separator to keep any liquid gasoline from making it's way into the vent line / canister.

Exactly. First time I've heard someone say that loop (AKA vent pipe assembly) is in fact a separator.

That truck probably wasn't equipped with a RETURN line, they started that business in 72. Your vent line appears to be wide open, and perhaps a tad dangerous? I recall someone recommending a valve of sorts for that situation, though post 7 looks like a good idea.