1974 Plymouth Satellite fuel vent line

Hello,
I just picked up a real nice 1974 Plymouth Satellite Sebring this weekend and I have a question for you guys concerning the fuel vent line running from the gas tank to the charcoal canister. I pulled and am replacing the fuel tank because it is full of rust. I can clean the actual 5/16" fuel line with no problem using forced air but when I blow air from the tank end of the vent line to the charcoal canister, there is blockage. The stoppage is what looks to be a "check valve" in the end of the steel line just prior to the rubber hose that connects it to the canister. When applying air at the tank, you can push down on the check valve and then air will travel through. To me, this is backwards. With the car running and the vacuum from the carb base plate pulling from the charcoal canister, shouldn't fumes and vapor from the tank be getting sucked into the carb to be reburned? Or is it designed so that once the car starts the vacuum in the canister pulls the check valve closed? This would then mean the tank would be vented through the gas cap while the car is running? It almost looks like a needle and seat in the vent line.

The way I understood (although 1973) is that while sitting, the tank vents to the canister.

When the accelerator pump activates, it opens the line to the canister, burning the stored fumes.

I guess I never thought about what happens to the vent line while running.

That would make sense. The needle would "fall" in the line and open the vapor path to the canister when the engine was off. My problem was that the needle was stuck closed (up) and on a hot day, the tank would pressurize so much it would push fuel out past the sending unit rubber seal. Cracking the tank cap would vent the pressure and stop the leaking. Not sure if my gas cap should be more vented than it is to prevent this.

Mine had a shuttle valve in the manifold (in front of the tank) that was sticking causing the cap to blow off when removed.
I had to drill that shuttle valve out.

I don't think mine had that needle valve up front, but it did have a relatively complex multiple point system to open the line passages based on manifold temperature and vacuum.

I "temporarily" have all that off and just have the vent line from the tank directly to the accelerator pump purge valve.

That valve should be the roll over valve. On later models its in the gas tank. Its there so if the car rolls over fuel cant run up that line and into the canister and all over the ground. It has a ball in it that seals against its seat when you blow air in it or if fuel ran up that line. The vapors from the tank will push right past the ball and get stored in the canister until the purge setup pulls the vapors into the eng and burns them. It will vent through that with the eng running since atmospheric pressure is pushing into the tank when running. Also the gas cap is sealed but if the vent line got plugged the gas cap has a safety feature so it will vent if a small amount of vacuum or pressure builds up inside the tank. Ron

Thanks Ron! That makes perfect sense! The free flowing vapors/vacuum pull from carb are not enough to overcome the weight of the needle but flowing gas in a tip-over situation would shut the line off.
Thanks again!