under pressure

[dav3]

Went to put some fuel in the car (Mk1) on weekend and when I took off the filler cap there was a hissing sound as air was sucked into the tank and I heard the tank clunk as though it was reforming after being pressurized, had a quick look and cant see a breather hole in the cap and car seems to run ok.
Don't want to leave the cap loose for obvious reasons but am worried that the tank might get damaged or might start to get fuel stave issues
Can anyone shed any light on the location of a breather hole or pipe for the fuel tank any help appreciated
Dave

[BGB autosport]

There should be a slight vacuum, If not you may fail your mot. I can hear mine hissing for a while after switching the car off, although by the time i have got out, shut the door, walked round the back and popped the boot its los the majority of its vacuum.

[Whitey]

your fuel tank vents via a black plastic pipe to the front o/s wing to a charcol canistor. This pipe often gets kinked or blocked, with a smell of fuel and poor running after brimming the tank.

A slight wooshing when opening the cap on an almost empty tank is normal, I guess if excessive you should check things out.

[dav3]

Thanks for that a few things to look at over the weekend in the sun I hope

[S6PNJ]

Mine always seems to be pressurised (more pressure in the tank than outside), should I be concerned? How do I go about checking the carbon canister etc?