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I noticed that when I take off the gas cap I hear a distinct hiss indicating that the tank is building up pressure, I always thought you did not want the tank to build up pressure, is or should there be a vent that lets the pressure vent out of the tank? I see a vent with a small air filter on it on top of the intake on the tank but it look like it is designed to do the opposite (let air in not out of the tank).
Thanks

Is it a hiss in or a hiss out....may be vacuum being pulled by the fuel pump?

No it is definitely a hiss out (pressure in the tank (, I had just installed a new fuel sending unit and the pressure was enough that some gas was forced out around the threads of one of the mounting screws. I took them out and put some thread sealer on them.

How the tank reacts depends on which gas cap you are using. They come in tow basic styles. Non-vented and selectable. Either works fine. The tank has tow locations it can vent. The cap and the vent valve. The vent valve will allow air in as the fuel pump sucks gas out. The vent valve will retain about 1/2 to 1 pound of pressure but will then relieve. So with a non vented cap you will often see a slight escape of slight pressure. Not a problem. If the vent system fails to allow air in the fuel pump will soon be unable to supply gas to the carb. Around 1955 the selectable cap came into use. It has a little tab inside and is marked for two positions. Fording (which makes the cap non-vented) and Normal (which makes the cap vented). This is the preferred cap and is best left in the normal (vented) position. Most M series trucks have a warning stencil near the cap warning you to remove the cap slowly since the tank may be pressurized.

Great post Wes as usual, much appreciated.

This makes since as I had just cut a new cork gasket for the gas cap and I bet it had been venting from there all along and the new gasket prevented it from doing so.

Yhe newer selectable vent caps are the best to use and are still cheap on the market as NOS.