I have a 83 f150. Every time I open the rear gas cap thereis a big pressure release.Where woul I look for the plugged vent line? Does this go to the front under the hood?

 

yes, there is a rubber line that comes out of the front of the gas tank and connects to a steel line that runs toward the passenger side, up the passenger side frame rail, and to a charcoal canister under the hood near the battery.