ff9,

Many times, people use 3 lines on the O.S. pumped engines. One is the fuel pickup, one is the vent, and the other is a filler that is capped-off with a fuel dot.

You could put a fueling valve in the fuel pickup line, but some of the fillers will be sucked open by the fuel pump and then allow air into the system, making the engine go lean.

Putting a 'T' in the line will force fuel through the pump into the carb, and then out the air intake. Filler pressure is high enough to do that.

With a 3-line tank, the vent must go from top forward inside the tank and exit the bottom of the cowl or fuselage. The filler line usually dumps at bottom forward in the tank.