Based on the Canadian military tanks, one of the tubes goes to a pipe within the tank that comes up into the small hemisherical dome at the top of the tank. This dome is a second part soldered (?) to the top of the tank that has three(?) small vent holes between the main part of the tank and the small compartment under the dome. My understanding is that this is an atmospheric vent with the second pipe being a short stub that lets fluid from the radiator enter (and return from) the tank.
