There should be a vacuum hose diagram somewhere on the underside of the hood, or possibly on the radiator support. Just about any vehicle made after 1968 will have some sort of filter on the gas tank vent, and a charcoal canister is the most common situation. A vent line will come from the top of the tank and run forward to the front inner fenderwell area. The flex lines from the tank to the frame can collapse, or the canister can get clogged to the point it can't vent pressure. Intake vacuum is supposed to go to the charcoal canister every time the engine starts to purge the trapped vapors out of the charcoal. Repeatedly topping off the gas tank until it's overfull can fill the canister with liquid gas, and keep it from venting pressure properly. If someone has changed carburetors and/or manifolds, it's very possible the canister purge line was not reinstalled correctly.
Jerry
