Getting gas to the carbs

[Guest]

I can't quite figure out how the petcock on my gas tank works. I understand it uses vacuum but I can get the gas to flow. Can some tell me if this petcock need to be in the horizontal or vertical position? I tried putting a brake bleeder vacuum on one of the lines to create the vacuum but still can get the gas to come out either one of the lines. There's not alot of gas in the tank but should be plently to flow out one of the lines. What am I doing wrong here? and yes I'm laying the gas tank down flat.