Gas Tank Ventilation/Collapsing

On my 73 Mach 1 I was on my way to work when I heard a popping metal sound and quickly realized that such a strong vacuum was being formed that the gas tank was collapsing. Upon removing the gas cap there was an in-rush of air and the tank made a pop as it expanded back to normal size.

I had a non-vented gas cap and assumed that was the problem. So today I got a vented gas cap and put in on the Mach 1. After only running for a minute or two there was already an intense sucking coming from the new vented gas cap. Unfortunately it wasn't enough, as the engine started to run rough almost immediately. Upon removing the gas cap there was still a massive vacuum holding the cap on.

Where else is the tank supposed to be venting? The gas cap doesn't seem to be providing it with enough air. I can't remember now if there was a vent on top of the tank, maybe that could have been pinched off or plugged?

The vent line from the tank should go to the canister? Where does is the charcoal canister supposed to be?

I just went and double-checked, I do not have a canister there. There are three bolts that probably used to hold such a device. What lines run to and from the canister? I don't have any lines dangling nearby.

Thanks for all the help so far, without knowing someone with another car to look at it, these kinds of issues are a real pain to figure out.

So, I found two steel lines that run together coming into the engine bay. They terminate near where the transmission meets the engine directly behind the air filter. I assume one of these is the line you were talking about that runs to the canister, but they are both the same size so I'm not sure what each one is. Do you have two lines running together like that? What should the other one go to?

Solved

I traced out those two small metal lines I mentioned: they lead to the transmission. Starting from the tank I traced the vent line down the car and found where it comes out in the engine bay. The line was hooked on to the vacuum tree coming out of the back of the intake. No wonder the tank was developing vacuum so quickly! It must've always been hooked up there since I got the car, and putting a new tank and filler neck seal caused symptoms to surface.

I have disconnected and plugged the line and the port on the vacuum tree and after a minute of running no longer hear the sucking air through the vented gas cap.

Thanks for all of your assistance.