Replaced the old cap with a new Slant cap($6.99). Still no Hisssssssss.:confused: "shrug"

It's a function of your gas tank emmissions system - on the Toyota Scion etc they have a pump that takes the pressure out of the tank from 2-5 hours after you shut the engine off. Pressure in the tank only occurs if the gasoline evaporates faster than the engine can suck it into the engine while you are driving. Altitude will definately affect the pressure in the tank as will temperature changes. I would say that if you took a quick ride on a bumpy road you would have pressure in the tank but if you down shift a lot then the engine vacuum may remove the pressure via the charcoal canister.