I could be wrong, but I thought that cap should be vented. A quick search of Rock Auto, what they are selling for 71 Road Runners are vented caps, several to choose from. Try that.

I think what happens is you are right about where the fumes go, but there still has to be some relief in the system to let air in to the tank as the fuel and fumes are vacuumed out. Read the description for the Stant 10807 cap, it sort of explains it.

Last edited by Kowal; 05/24/20 12:06 PM.
'69 Hemi Charger 500, ‘70 U code Challenger R/T
(These and a bunch others at www.dkowal426.com)

P.J. O'Rouke: "The old car ran perfectly, right up until it didn't."