I "vented" my reproduction cap with a 1/8" drill bit through the plastic center on the inside of the cap. No more belch of fumes and no more giant sucking sound.
The new rubber (probably PVC) gaskets are real tight on the screws which prevents leaking.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission