Looking for an education in fuel tank venting

The deal with HOW the tank is vented goes back to the early days of emissions controls.

The tanks are all vented, otherwise there would be pressure issues on hot days. The early cars vented through the fuel cap. In the late 60's the government started clamping down on fuel vapor emissions, so the automakers had to come up with a way to capture fuel vapor instead of releasing it into the atmosphere.

So, the first effort was the "4-corner" tank venting that your '71 and some '70's had. Each corner of the tank has a vent, with 4 lines leading to a vapor separator in the trunk next to the filler pipe. From there a line led forward to the engine compartment where a rubber line connected to a port on the valve cover breather. There, the excess fuel vapors were drawn into the engine crankcase and eventually burned.

That was a complicated system that probably didn't work too well, so in '72 the tank was simplified with one vent leading forward to the charcoal canister in the right front of the engine bay, which also collected vapors from the carb.

So, that's the deal with the vents. My car? I wouldn't worry about the two extra gallons.