Zerodefect

Think of a straw. A fuel tank is alot like a straw filled with water. If you keep your thumb over the top of the straw no water falls out of the bottom. If you pinch said straw and pull your thunb off, water will flow slowly.

This is why that pressure line is critical. Air deosn't like to flow through pipes at low pressure. ("breathing through a straw") The longer the pressure line the more friction resistance that air has. If fuel can't flow from a sealed tank (straw with thumb over it), you have to let air in.

The reason the pressure line is on the pipe is becuase it prevents fuel spills when upside down, it prevents dirt from clogging your pressure/breather line because their isn't dirt in the pipe, and maybe you can get a little exaust pressure for more constant tuning.

Real cars, old ones at least, used to have breathers on their fuel tanks, When you flipped over your old jeep all your fuel would pour out of the breather. Now real cars have an charcoal canister or sealed evap system that allows air into the tank so fuel can flow but contains the fumes.

I tried a longer pressure line on my old RC8 like one of our faster racers. It flopped loose and hit the flywheel. The flywheel cut a hole the size of my thumb in the pressure line. The only thing I noticed was my car was running slightly leaner from mid to top end.

In airplanes we can't reach the fuel tanks as they are buried in the air frame. We would pump fuel into the tank by poping off the muffler line and pumping into the pressure line to fill the tank. We often forgot to put the line back on, or it got worn and would fall off in flight. Top end got a little leaner, but not much.

Seriously, hook a manometer to your pressure line and rev your engine (works best if you have a load, so hold your tires screeching on the ground with a good amount of throttle or use a prop and an engine stand. Your not going to see much pressure.