DES57, Yes, that's the way most vented caps work, with pressure change to either let air in or fumes out. But with a small enough hole fumes wouldn't be a big issue in the garage, IMO. It only vents fumes as the contents inside warm up.
I once collapsed the tank on a boat when I forgot to loosen the manual vent screw on the cap. Running along just fine and heard a "BONG". Looked back and the tank had a big kink along one side. I don't know if the fuel pump on a 47 could generate that much suction or if it would starve the engine for fuel before the tank collapsed. But it's important to have a proper vent.
