This I pretty much the exact same system I came up with except i was gonna put my fitting in the top of the tank. But I decided it would be simpler to use a vented cap. What's wrong with a vented cap versus this?
From what I can tell, the vented cap lets pressure out but no air in. You need to get air into the tank to get the gas out. If you have suction when you open the cap your tank isn’t vented properly, unless you have a completely stock system. If you decide to vent the tube, like I did, make sure you put it as close to the top of the tube as possible. So when you fill it the fuel pump nozzle goes in pass the vent.
