I'm using a petcock valve fron a ATV and it has a reserve. Mine work's great, it's not really a reserve....it just let's you know you are about out of fuel.

It's just a long screen inside the tank with a top inlet and a bottom inlet...the top inlet being the "on" position...when the fuel level goes below the top inlet, you run out of fuel and turn it to the "reserve" and starts to use the fuel from the bottom inlet...