The fuel return line does come off spill rail back to tank.
The reason you probably have air in the fuel lines is because you've got clear hose pipe and not proper rubber/silicone braided fuel lines,when the engines running at temperature them type of hoses tend to soften and let air in around joints and then reseal themselves when cold,had a very similar problem on my p38. Install the correct fuel lines from fuel filter,{make sure you drain out any water in the bottom first} and it should run better and start easier. You could also install in the line a one way fuel filter valve just so you can keep an eye on it.