Re: What is this for exactly (pic)
I believe that large nipple is the tank overflow, and should drain to the ground or maybe to a container. If it goes to a T, I guess I'd follow both ends of the T to find out where they go.
Put a hose back on that nipple, and blow through it - air should come out of the little hole right next to the filler hole under the gas cap. It's meant to allow overfilling to drain through that hose instead of all over the outside of the tank.
I believe that small hose is the tank vent. You should be able to blow through that one also, but it will be kinda hard, maybe impossible unless the gas cap is open.