Fuel problem! please help

I own a 89 gti cabriolet with an 1800 8v engine and im trying to fix a problem with the fuel system.
Recently every time I remove the fuel cap there is an almighty hiss and what sounds like the fuel tank popping be into shape, as if there is so much suction it is being destorted.
Could it be the tank breather and gravity valve setup and does anyone have any pictures of what i should check?

Yes, for late cars the hiss is a sign on a blocked gravity valve. It is mounted up in the rear arch behind the plasitic liner. Your other option is to fit a filler cap with a breather in it but not as safe in case of an accident.