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The fuel filler cap has a 1 way valve in it to allow air in to prevent a vacuum
but yes the purge system vents tank pressure into the engine inlet so prevents it building up on a hot day which can be dangerous
so just blanking off the tank vents probably isn't a good idea
in the days before purge systems the tank was vented through a charcoal cannister and out to atomosphere under the car
and before the days of charcoal cannisters the tank was just vented and breathed through the cap
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