The way it is supposed to work is that the non vented top tank creates a vacuum and pulls the fuel up from the vented bottom tank.    We had 2-105's with this setup.      

Our 1st 2-105 always worked fine other than you couldn't fill the bottom tank unless you were going to start using the tractor immediately, if you filled it and let it sit the bottom tank would overflow from the top tank.    The 2-105 we have now was bought used and has an electric pump on it to pump the bottom tank into the top tank.   It works but I often forget to turn the switch for the pump off until I see fuel bubbling out of the cap for the top tank.

Our 2-135 and 2-155 have the top and bottom tanks hooked together (no cap on bottom tank) and the engine pulls the fuel from the bottom tank, IMO this is a much better setup.

According to Bobby Grief there were a couple years of 2-135/2-155/2-180 tractors made before they started hooking the two tanks together and only had one cap.      He says a lot of the old style were converted to the new style as well.