You're not hearing air escaping, you're hearing it rush into the container! If the container is crushing, it means the air that was inside the container is being absorbed by the liquid. The container is tightly sealed, so the external air pressure is crushing it to take up the space left by the disapearing air.grendels_arm said:
I don't know why the air in the container is being eaten up, but it's probably some kind of oxidation process of the fuel. The fuel is changing (probably going stale). I've left gas in a container for over a year, but I've never seen the container crush itself. It may be that my container has a valve to allow air in.