Hows this for a thought: Propane is a hydrocarbon fuel in a gas state. Hydrocarbons have the ability to permeate through the flexible rubber gas lines on our trailers. I believe the hydrocarbons in the gas simply pass through the line into the air and what is left is just a gas that no longer has any ignition properties. You can't see it happen, but it does. Engine manufactures have spent lots of time and money addressing this problem with gasoline. Gasoline is just a hydrocarbon in liquid form. Same hydrocarbons, same effect.