I was driving home from the dyno today in my dads 86Z and when i pulled into my driveway and turned the car off, I heard a loud hissing sound, like I tire leaking. When I looked under the car, I found the gas tank vent spewing fuel vapor. I then loosened the gas cap and to relieve ALOT of pressure. What could hav caused this? I have never seen this before.

 

This is normal with the stock exhaust system located under the fuel tank. It relys on air flow under the car to remove the heat from the muffler. On a dyno this doesn't happen. Gas will boil in the tank from the heat. GM discontinued the 305 L69 in '86 models due to "fuel boiling".

 

the vent valve is a one way valve. air in only. replace it before it starts to leak.

its function is to let air into the tank as gas exits. you can get it at a gm parts dept. for around 15 bucks

 

I was thinking that a vented gas cap would work. What are the pros/cons of doing this?

 

do it right it is cheap!!!!

 

You should do both. When the tank builds too much pressure, it will come out somewhere. It is suppoased to come out the charcoal canister, but it can only handle so much. I would run my car hard and park it in the garage and then listen to it humming and moaning for 15 - 20 minutes out the gas cap. It also makes your gas mileage stink too, as a portion of your gas never gets to go to the engine.
The only long term solution is to get the exhaust (muffler) out from under the tank (especially if it is a flowmaster). It worked for mine.