OK, quick punt on the fuel tank thing. As I said in my initial post, the tank is SUPPOSED to run under positive pressure. This I have read. That's why you get the hiss. It's normal. But if there is too much pressure for any reason, there needs a safety valve, venting outside the tank. Clearly you also need to let air in to replace the fuel used, but running under pressure (which helps to push the fuel along) and letting air in as well is a fine balance I guess. What you don't want is negative pressure at all. So if you use fuel faster than you can get air in you starve the pump. I would hazard a guess that the cap allows pressure to build up from fuel vapour and as the tank level falls it keeps a slightly +ve pressure negating the need for fresh air to be drawn in. If there is pressure, you don't need new air - right? This way you'd probably get a bigger hiss when the tank is empty. However, if insufficient vapour is produced, say during cold weather or on demand by a 4.2 lt engine during acceleration then fresh air needs to be dragged in quickly to allow fuel to leave. Not under pressure as it would prefer, but at least without drag as it were. So I reckon the the cap lets pressure out and the breather lets 'emergency' make up air in. That's why it's filtered. If it gets blocked - ('cos it sucks in dirty air all its life) then the tank gets a negative pressure inside.
OK that's my rationale.
Chris