S&B Tanks are filled by releasing air through the main 3/4" vent(red) as fuel goes into the tank. When the fuel level hits the level of the vent, air can't escape which causes the fuel backs up into the filler hose. When the fuel hits the nozzle, the nozzle stops filling.
The orange vents do NOT control filling. These are Roll Over Valves that have a small 2mm hole. These are designed to vent during the operation of the truck.