The vent can is located behind the back seat and protrudes through the back panel at about the height of the fuel tank. Has a thick rubber gasket that seals the cab and prevents any outside fumes or water from entering the back of the seat area. The canister has 4 short steel lines extending from it that corresponds with the lines on the original style fuel tank. The factory original cars had a small collector mounted to the underside of the trunk above the canister and held the lines going to the canister clamped up and out of the way. There are 3 lines close together and a 4th spaced just a little bit away. The 3 lines close together connect to the 3 lines coming from the tank and the other line vents. I blocked that line when putting my car together and it pressurized my tank even with a vented cap. when I removed the gas cap for fuel-----Whoosh!!! Every time. I finally found my mistake and unplugged that line. Problem solved. This is on my 71' Skylark.
When a gas cap is designated "vented" it means that the cap will allow air into the tank, but not allow vapors out. The 1972 caps are vented in that way. Fumes are routed to that canister behind the seat, and onto the charcoal canister. The 70 tank is vented to the atmosphere with 2 lines that extend upwards from the rear passenger side corner with a plastic breather element. It uses a non vented cap.
