1997 Toyota Celica. Fuel leaks when gas cap is tight.

[miguelr1997]

I have a 1997 Toyota Celica ST 1.8. About 2.5 years ago, I had my gas tank replaced since it was leaking due to a car stereo dealer that installed a power amp in my car at the time, drilled 2 holes under my back seat, that punctured holes in my gas tank.
Sometime last year, when the fuel gauge on my dash read less than a quarter tank, I noticed the smell of gasoline. I looked underneath and noticed a small fuel leak trickling under the center towards the passenger side of my gas tank. I loosened the gas tank cap and noticed a strong whooshh of air pressure that lasted for about 3-5 seconds. I went to the gas station and filled up the tank full of gasoline and noticed that there was no leak. After driving for a for about a week, I noticed the smell of gasoline again and the fuel leak when my fuel gauge on my dash read less than a quarter tank. Again I noticed a strong whooshh of air pressure that lasted for about 3-5 seconds when I loosened the gas tank cap.
I then decided not to fully tighten the gas cap when refueling and there has not been a fuel leak ever since. I've been driving the car with the gas tank cap not fully tightened for about 1 year now. Obviously there is something wrong and I know that I shouldn't have to loosely tighten gas cap to prevent fuel leak. The check engine light never turned on, which I read somewhere can occur if driving with gas cap loosely tightened. What can possibly be wrong? Can it be the evap system? I would greatly appreciate some expertise response so that I can take some form of action to get this issue resolved ASAP.

Thank you