One of the problems I encountered on my little test ride across Texas was that fuel would not transfer from aux cell to main tank unless the cap was open. Venting problem. I have learned the tank can and usually does operate under some pressure. Pressurization is not requisite but rather a byproduct of the fuel pressure regulator constantly shooting fuel from the fuel rail back into the tank and the fact that the oem cap is equipped with a flapper valve on the vent line to let air in but not out and is designed to close in a tip-over. Here's the vent line at the filler, the small opening between 4 and 5 o'clock. The larger port around 10o'clock is the overflow.
These quick connect fittings seem to be excellent quality. The two fittings (must be ordered separately) and shipping cost around $25.