Air getting into fuel system

[Almo]

I seem to be getting air into the fuel system on my '07 Connect TDCi which makes the engine cut out about 5 seconds after start up at idle speed. If I rev it up the instant it starts then that gets the fuel pressure primed up quickly enough to keep it running. Once the engine is running its fine. After sitting for about half a day though the air is back in there again.
Any ideas on how to detect where the air might be getting in? A visual inspection of the fuel lines etc has not revealed anything.
I did replace the fuel filter about a month ago and the three fuel line connectors were really tight and it took a lot of force to get them off. Wondering if maybe some damage happened then. But I've done 700kms since then trouble free. Hmmm.
Any assistance much appreciated.

[Altransit]

Fuel filter is the most common place to get an air leak, but another place to check is the seals on the leak-off pipes where they fit into the injector. It may be worth replacing them just to eliminate the possibility

[Almo]

An update on this: The air must have worked it's way out of the system as I can now start it with no throttle and it keeps going.

Glad I waited and didnt start replacing fuel lines and such like. The air must have got in there when I replaced the fuel filter. The reason i didnt rush into anything was I worked out a 'get around' by giving it some throttle when I cranked it over so that it revved up when it first started and primed itself before it could die.
Never stop learning eh.