Overfill valve in vent line ?

67Dart273 said
I don't have a 75 shop manual, so I don't know the specific plumbing / hardware differences in the system, l but the basics are in most any shop manual after 70 or so. You can download a 72 here

http://www.abodyjoe.com/pictures/Misc. car info/1972 Plymouth Chassis Serv Man.pdf

You have to read both the fuel section and the emissions section

Basically, the tank either has a separate or built in vapor separator. The only place "out of" the tank for vent is the line in question.

The filler caps have never been vented, and if you look at those threads, you'll see how the older cars (pre 70) had a vent welded into the upper filler tube. The caps have a "pressure vaccuum" relief.

That line comes up front and ties into the carbon can system. The ball valve in the end of that line is supposed to keep liquid surges of fuel at bay during filling, etc.

The MAIN purpose of the line is what I've been trying to convey. It IS the tank vent. The reason it is there and the reason it ties into the carbon can is to recover fuel vapors instead of venting them overboard.

Thanks for the input, I think I am finaly making sense out of this ....
I understand the vent line allows recovery of pressure let-off or fuel vapor could push out the cap. I do have a 74 manual as well as a Chiltons. been studying these as well as the pic from your post. It looks like the 'valve' has a small tube in the center, which must be the part that according to the manual - open at 1/2 PSI differential to allow vapor escape. Chilton's includes this under the "vapor saver system" so I guess it's supposed to conserve fuel by limiting free evaporation with a very slight backpressure. That small slider is enclosed in another larger slider that will close off the end of the tube when it lifts. I guess that is what would stop a liquid surge as you mentioned in last post.
So, that pretty much covers how it works. I guess the answer to my real question of what happens without it is - not much. I suppose since the lines go to the canister the fuel is recirculated anyway and it's not a big loss?
As long as I can keep it in a vertical position; I think I will just cut the valve off the old tube and shove it up the hose ahead of the new tube end. Should work as it was intended.