Took OB out yesterday for a 50 mile spin. It was about 85 degrees F outside. Stopped at a station to put some fuel in. When I turned the cap, a lot of air rushed into the tank, or maybe it was rushing out. I'm not sure. Checked the aux tank which was empty and the same thing happened there altho not as much. My 454 system does not have an electric fuel pump, depending solely on the engine fuel pump. The charcoal canister system is long gone. There is supposedly a vent on top of both tanks.
My understanding of vented gas caps is that they are to let air in but not out. I'm concerned that with so much air rushing in when I opened the cap that my tank is under a partial vacuum that will make the pump work harder. Thoughts please?
You Know Who
YKW

The cap that I use is a NAPA/Balkamp #703-1033.

This cap will allow air in but not out. It wasn't real tight so I added an extra rubber washer for a better seal