Fuel tank venting

[HondaWalt]

I know this subject has been beaten up already but I still have questions. When riding for a while and then stopping, I shut down the engine to fuel up. When I turn the fuel tank cap (I find what I think is a positive press sound release). The cap is original with the bike (1980 GW 1100) and secures correctly. There is no visible sign of a relief vent hole anywhere... is it like a radiator cap where the sealing plug over comes the spring when the pressure is too great?
Where is this pressure coming from?...if it's the fuel pump how?
It only has a one line into the pump and the other to the carbs? The machine runs with no problems at all with this traped condition (pressue/vaccum) and lack of an obvious venting hole...should I be concered? You know... don't fix it if it ain't broke! Has anyone else encountered what I have here?
HondaWalt

[virgilmobile]

Every Wing I've owned has the same positive pressure during the day.its a result of expanding gas and is normal.If the cap were plugged there could be a vacuum in the tank(bad).
Having a open vented tank is of no benefit other than gas fumes.Not something I enjoy smelling.
The gas cap does have a one way valve in it,air in when cool,no vapors out when hot.

[wjnfirearms]

Virgil pretty much got it right, as usual. Unless you are having any fuel delivery issues or the pressure built up seems way too intense, I wouldn't be concerned. It's just a normal occurrence as the design of the fuel tank and cap holds most of the vapors internally and the hotter it is, the more gasoline vaporizes as it's evaporation temperature is lower than water. Only fuel injected vehicles use positive pressure within the fuel tanks, and that's more for emissions than fuel delivery. Then again, my Harley is fuel injected and the tank pressure is very minimal. Cars tend to be a bit more so due to size.

[HondaWalt]

Thanks guys for answering my question... I read other blogs about venting problems, the end result was drill a vent hole in the cap to resolve the problem which led to my confusion (the original cap is vent hole free).
HondaWalt