This old car makes be feel ignorant. Basic Questions inside.

Unless the previous guy "did something" that capped line is the ONLY vent for the tank. Here, download a free 72 shop manual, and thank AbodyJoe. You have to read a combination of the fuel section and the emissions section in the back. The section on the vent line from the tank to the front is page 25-14. Page 14-2 has some general info, and page 14-76 and onward has more

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

This thread

http://www.forabodiesonly.com/mopar/showthread.php?t=183956

The filler CAPS are not truly vented. The caps used on the later cars with the carbon can have a "pressure vacuum" cap which means it has relief valves to let in air or let air out at a certain pressure level. They were meant as a safety, NOT as the primary vent system, this thread:

http://www.forabodiesonly.com/mopar/showthread.php?t=202184