If you're drawing a vacuum at the fuel bulb, there's one quick way to eliminate the tank vent as a source of the problem: crack the fill. If the vent is plugged, and you crack the fill, any vacuum in the tank will be eliminated, and the bulb will retake it's normal shape. If that happens, you've probably got a plugged vent or vent line. If not, you can focus on other parts of the fuel delivery system, as noted in the other posts.
A simple way to check out the selector valve is to switch the two feed lines from the tanks, at the valve, so that selecting Port on the valve really connects the starboard tank, and selecting Starboard connects to the port tank. If the condition reverses, you know the selector valve is working, and that you have a problem getting fuel from the tank, to the valve. So, you'll need to take a look at the pickup inside the tank, and the feed line from the tank to the valve. Otherwise, your problem is the valve.
Likewise, if you disconnected the feed line from the problem tank at the valve while there is a vacuum at the bulb, and the bulb still shows a vacuum, then the problem is a blockage at the valve. Otherwise, reconnect the feed, run the motor some more, and the vacuum should occur again. In that case, the problem is the pick up or feed line. Chasing this back a little, if you've determined that the valve is not the problem, then disconnect the fuel feed line from the top of the tank, while there's a vacuum at the bulb. If the bulb retakes its shape, you should focus on the fuel pickup inside the tank. Otherwise, replace the fuel line between the tank and the valve.
No matter what, don't blow compressed air back into the tank. It's a very bad career move.
Good luck!