You might get your car up on a lift or jack stands and try to follow the 3 lines...I have a 1974-450-SLC and it has 4 metal lines that appear about 2 feet in from of the rear driver's side tire and continue towards the rear...
One of them is the brake line and can be followed back to where it tee's and feed the 2 rear caliphers...
One of them goes back to the gas tank and is the return
One of them goes back to the fuel pump/filter and is the fuel supply
One of them goes back to the gas tank and I am just guessing that it is a vent...where the 4 lines first surface in front of the rear tire their is a something kind of shaped like a figure eight and I think this might be a vent but where it comes from I cannot say but it is one of the 4 lines and can be traced back to the tank...Does anyone know???

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James A. Harris