failed inspection due to gas pressure leak

These are leak places, but there is also the charcoal canister and the rubber hoses that connect to it. Yes, a low source of air pressure, listening for a hiss, and if that doesn't work, spray bottle of your favorite 409 or window cleaner will work. You need up to 12psi (as in not to exceed), so, a large balloon and a string tied around the end on one of the hoses from the charcoal canister? I say start with the condition of the hoses on the canister first.

What vehicle are you talking about? Charcoal canisters have been around since the very early 70s. At the same time, the older cars, like my 68 Charger, the vent tube goes up into the C pillar and makes a hook near the top and then comes down to the frame rail and just sits in there, so it is open to the atmosphere. When they get plugged, they tend to vapor lock the gas tank and starve the engine of fuel, so you have to pop the gas cap, so go figure. The charcoal canisters are usually in the passenger side corner of the engine compartment behind the headlight area, big black plastic container with three or so rubber hoses coming off it, if it is missing, that's where the leak is.