Non-Vented gas tanks? HELP!!!

I did a search but nothing i found exactly answered my question. I'm working on an OT truck for a friend of mine and it came from the factory with two tanks. It appears to be built from several different year models as there are some emissions pieces where they shouldn't be for that year (1974) and other stuff missing that should be there. Anyway, the aux tank has been the only one used for years and i'm trying to get the "main" tank up and working. The Aux tank has a vent line to the atmosphere. Themain tan has no vent. (it does have the little second tube next to the gas filler to help when filling up but it's not a vent to outside of the system when the cap is on)

I assume a "vented" gas cap is all i need to be safe?

Thecap came in a kit and looks to be vented (a small plastic "bump" in the center that if i suck on it {no comments} it pulls air in.

Should there be a slight bit of pressure in the tank when i've driven it and open the gas cap?

eack tank has it's own filler neck and cap.

If you had a non-vented tank and a non-vented cap, what would happen?

would it just overcome the ability of the fuel pump and run dry?

could it cause any danger of leaks, explosion, etc?????

 

Tell you from experience, no vent, non vented cap, it will suck the tank flat! 1979 chevy truck with a jake leg aux tank. vented cap should not allow any suction in the tank or pressure. no sound when you open it.

 

Thanks for the reply 26 Roadster.