Abstract: A vent for a truck mounted tank comprising a duct, one end of which originates in the normal upper expansion space of the tank interior and terminates on the outside of the tank so that the duct can convey air from the interior of the tank to the outside atmosphere surrounding the tank. The duct is so designed that regardless of what position it is in, it has a generally upward or vertical component between the inlet and outlet portions of the duct, which provide that a portion of the duct is always above the level of the liquid in the tank, which prevents liquid from flowing through the duct from the interior of the tank to the outside atmosphere regardless of what position the tank is in. The duct preferably extends the full length of the tank and completely encircles the tank interior at least once. To prevent liquid from inadvertantly entering the duct, the duct may be provided with a low pressure check valve.