Pressure in Tamora Fuel Tank

[willy mussell]

When I remove the filler cap I notice that alot of pressure has built up in the fuel tank - it can sometimes hiss away for about 15 seconds before I can start filling. As the car has run perfectly for the last 10,000 miles, I assumed this was normal.

However, during this hot spell, I noticed a smell of petrol in my boot - no sign of petrol just vapour. I checked around the filler pipe and noticed a breather pipe.

Has anyone else noticed this pressure build up?? If so how can it happen with a breather pipe??

All responses welcomed