Vented -vs- Unvented Gas Cap 72 Scamp w 318

The 72 should have (unless it was removed) the emissions control vent system, (Evaporative Control System) known by many as the "carbon can" These used a "pressure vacuum" cap which had a pressure release at very low pressure in BOTH directions, IE when the tank develops "so much" positive pressure the cap relieves, and the same is true for vacuum

In NORMAL operation, the carbon can vent system should take care of venting the tank and the pressure vacuum cap should not need to relieve. Look it all up in the shop manual:

http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual,+download

The 72 manual

[ame="http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf"]http://www.abodyjoe.com/pictures/Mis...Serv%20Man.pdf[/ame]

Earlier cars, like my 67, have a SEALED cap. The vent line on those is welded/ brazed into very near the top of the filler tube, goes up high in the rear fender in an upside down U, and then travels back down through the floor of the trunk, where the open end simply ends up inside the rear frame rail.