Gas Tank Vent

Not sure how to answer that. The reason for 4 lines going to the tank in the trunk was to do exactly what it's called --a liquid vapor separator. The idea behind the 4 lines out of the tank was that at any given time at least one of those lines would be open ---above liquid.

THAT is the vent so far, at the time.

So the "vent", goes up into the vapor separator, where any fuel "slop" can drain back into the tank, then the vent goes up front.

It doesn't really NEED vacuum on it, is simply needs to be OPEN

So if you are eliminating the carbon can system, you need to replace the "up front" line with "something" that goes up high in the car (clear to the top of the trunk) then back down and out through the trunk floor --left open.

If you look at the diagram on one of those threads of the pre--70 vent system, they essentially use the fuel filler tube for a "liquid vapor separator" by cutting the vent line into the filler "nice and high," running the tube as high as possible in the rear quarter, then finally, back down along the filler, through the trunk floor (gasket) and simply open - ended in the rear frame rail.

The whole deal is, you need some way of guaranteeing that do not start a siphon. Let's say, you eliminate the vapor separator, and plug all the tank lines except one. Let's say you run that line up high, and back down, open, below. There might just come a time when.................

you park with that side "down" on a sidehill...........

A nice hot day expands a full tank of fuel..............

and a siphon starts, and when you get to the car, you either have police and fire, a smouldering pile, or at least, not much fuel left.

Now, the OLDER system, with the vent in the filler tube can ALSO siphon under extremes, IE the tank very full, but the MOST it can siphon, is maybe a cup? or so, before the fuel gets below the filler tube vent tube connection. As soon as the level goes below that, the siphon breaks.