Aux. fuel tank venting question

[Tagalong]

Several of you may recall you helped me with the installation of my combo in-bed fuel tank. Well, I just completed the installation but would like your expertise on what I believe to be proper venting of this set up. I plumbed and wired the system but plugged the tubing ends and put about 15 gallons into the tank to check for leaks prior to connecting to the truck vent and fill tubes. After welding a barbed male connector onto the Ram's vent and fill tubes, I went to connect the tubing to the connectors and fuel spilled out of the vent line. I realized the fuel is simply the "splash" of the moving of the fuel in the tank getting into the vent line.
Here's my question.... (As the tank is mounted in the bed) There are three ports on the drivers side of the tank. The bottom one is a drain, the front most port has the draw tube inside for the supply but the port next to the supply port is my question. I've set it up as the vent, and connected it to the vent line of the truck. When the Aux tank is full I'm not sure I really want that much "splash" getting into my OEM tank (the fuel also bypasses the Aux. tank supply fuel filter). I almost think that port may be for a return line. As I'm only pumping from the Aux. tank to the OEM tank I don't require a return line. My question is, does the Aux. tank vented cap provide enough venting for the tank when you are pumping to the OEM tank? Can I simply block that port or should I put a one-way check valve in the line to stop the "splash" yet allow greater venting when pumping to the OEM tank.
FYI I used ALL of your suggestions...
I installed a simple (metal) in-line fuel filter, then the switching valve next to the Aux. tank in the bed. Went through the bed and plumbed the fuel pump and a large NAPA diesel fuel filter, then into the fill tube. It sounds like alot but what is below the bed floor fits in a neat and snug package protected from road garbage.
Thanks for your help.
Taglong