Not sure what either of you drive but by the late 70's with charcoal canister etc the caps are vented. The early type seal completely like a valve cover cap with fibre gasket to seal. The later models are the same as now, flip them over and they have the plastic pressure release valve inners. These can fail over time.
It's good to check on the condition of your hose connections above the tank and on the sides above the exhaust. These can be perished and weeping but not enough to drip.