I've read some members drill a small hole in the gas cap to relieve the pressure, seems like others have had a similar issue with locking fuel caps? Wouldn't this just be a bandaid though instead of fixing the issue, and fuel could potential leak onto my paint? Maybe this cap is faulty?
yes, that would be the correct action. However, if your car is a '73, it came with a valved cap that only opens if the pressure in the tank is around 1 psi. With a vented cap, if you do not fill the gas to the top of the filler tube, fume pressure should activate the valve in the cap and release the fumes into the air. There should not be any liquid involved.
I suspect you may have two problems. First the '73 models have a vapor separator in the back that collects vapors off the top of the tank inside, separates the liquid contained with the vapors and sends it back to the tank while routing the vapors on to the front of the car to the canister. Normally, there should not be much pressure in the tank on a hot day because the fumes that make the pressure are going to the canister. I think there is a valve in there that releases the pressure and it can be stuck from what I have read here.
If your canister is connected to the carburetor properly, fumes remaining after passing thru the charcoal then get pulled into the carburetor.
I also believe that if your carb is flooding, then some of this fuel can drain back to the canister. Given my car has no canister, I have not paid much attention to how it should work.
Your shop manual will give a more accurate description as to correct function and hose connections.
If you loosen your gas cap and the problem goes away, then you can figure the main problem is in the tank end of the equation, most likely.
If you look down the carb while running and see raw gas going into the intake, then you can figure you have a carb problem most likely, or maybe too much fuel pressure which is not likely, I suspect.
