After driving my 88 bronco for a few miles I start smelling gas on the outside around the cap and noticed the tank is slightly pressurized so I need to know how that system works. I presume the cap is going bad but since everything is original some other parts might need attention too.

 

There is a vent on the top of each tank (as gas expands it pushes fumes out the vent). They connect to a steel line that crosses over to the passenger side under the back of the cab (on a truck). It continues up the pass. side frame to the charcoal canister under your battery. The charcoal canister filters gas fumes; but it would load up pretty fast so you have a line that goes to the purge valve which plugs into the side of the throttle body. When the motor is warm enough and operating at the right speed the purge valve will open so the motor can suck fresh air through the charcoal canister and clean it out.
Maybe your vent is clogged, or you may have a leaky fitting on the fuel line ? and the filler cap area is just where the smell comes out ?

 

Loose clamps of filler hose.