It would seem like gas caps need two valves to maintain proper pressure. One to relieve over pressurization do to expansion and the other to eliminate negative pressurization when engine fuel pump is running. Just a hole in the cap allows vapors to escape, although minimal, still a dangerous combustible in a confined garage.

1957 Chevy 3200
Daily Driver
PS, A/C, Tilt column, Rebuilt 350, Rebuilt TH350, Reupholstered Bench Seat, sound proof/insulated, LED headlights/taillights/backup lights/interior courtesy lights.
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