How does a gas tank vent?

[trucks72]

Not the 71-72 gas tank. The 67-70 tank. If you take a gas cap, turn it upside down so you are looking at the bottom of it. Blow on the center. It will not blow out. Now suck on it and air will come in. So when pressure builds in the tank where does it go?

[Zeke]

Might be wrong but pretty sure it stays in the tank until you take the cap off, outside temp changes or use gas driving.

Even new cars don't vent to the outside that's why you get a woosh sometimes when you take the cap off. The canister on the 71-72 tanks and newer vehicles have canister to hold the gas vapor until it can be sucked in by the engine. The newer vehicles have a fill vent because when they went to unleaded gas they made the filler smaller to keep people from filling with leaded gas. Some gas stations have nozzles with contraptions, California, to suck up the fumes while filling up.

If you can blow both ways on a vented cap you probably need a new one.

[LONGHAIR]

The "vent" is not there to let the pressure out....it allows air to come in to fill the space when the fuel is drawn out by the fuel pump. If it didn't, you would soon have a vacuum inside the tank. Once the vacuum level inside the tank got above the fuel pump's ability to pull......ya got nothin'. The engine won't run, or you have crushed the tank.