It's been a long time since I've been into one of them, but I believe the top line is a vent, which should be attached to a tube going to the outside, to allow air in as the tank drains, and vapors out. (Long before the days of carbon canisters and closed fuel systems!)
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!