[Mike_The_Grad]

I think 70-72 were the only years that the EEC tanks were used. I have a 72 c10 with 350/th350. Because I don't have to smog my truck I removed everything I thought at the time was related to it.( I was only 18) that included the evap canister,and associated vacuum lines. I still had the hard steel evap line connected to the tank, but it was just open to atmosphere in the engine bay close to where the fuel lines connect to the mechanical fuel pump. But I still had the original non-locking gas cap. Which is vented. But it's considered to be vacuum vented. Meaning it allows air in while the fuel is drawn out but the fuel pump. If there is any positive pressure build up in the tank, then that is where the evap canister comes in to the equation. The evap canister stores the excess fuel fumes while the engine is not running and releases them when the engine is running in order to be drawn into the intake/carb.
Anyways, I wrecked the truck back in 2004 and it sat for about 8 years. It only ran a few times in those years until I could finally invest some time and money. Well the original gas caps rubber seal was shot,which allowed water to get into my tank each time it rained. So I got a chrome Stant locking gas cap, pulled the tank from the cab cleaned it out real well, which surprisingly was in near perfect shape on the inside. Replaced the sending unit, which for the correct setup has two outlets: 3/8" main fuel supply and a 1/4" fuel return line from mechanical pump.
The vent line that would normally go to the vapor can is actually ran along the frame rails parallel to the fuel lines, enters the cab at the same place as the fuel lines, but splits off along the bottom edge of the fuel tank towards the passenger side rear cab support pillar up the inside of the pillar and loops back down to the top passenger side of the fuel tank where it enters the tank and runs along the inside top to about the middle of the tank.
Takes longer to type and read than to visualize.lol.
So I eliminated the vapor line that ran along the frame rail and basically have my tank vented to atmosphere below my cab. With mine setup this way it doesn't matter if I ran a vented gas cap or not because there is always a "vent" to atmosphere. But if you retain all the components of your system,(which I recommend) then you want a vacuum pressure vented gas cap.