Air in fuel system- weird case!!

Hi,

I had trouble starting my 1.8TD Mondeo 1998. I found out that if I cranked the engine immediately(without giving the glow plugs a chance to warm the cylinder) for a few seconds untill it nearly caught, then switched ignition off and then ignition back on and let the glow plugs warm the cylinder, it then started first time. I reckoned that the problem was air in the fuel pipes and the engine cranking was clearing the air and priming the pump. So I changed all the engine bay fuel hose to clear stuff but couldn't see any air in the system. I even bypassed the heater and filter and used a small clear filter - still no sign of air, but still I had to give a few seconds cranking before using glow plugs and starting. By the way it start first time when warm.

So what can it be? All I can think is that it's either the fuel pipe coming from the tank letting in air - but surely if there is no air in the pipes near the pump it should still start quickly?
Or it may be that somewhere inside the pump there is an air leak which letting air in to some area of the pump!
Can the injector pipes be letting in air?

Glow plugs are all fine getting 13V and loads of amps and glowing bright red.

Any help or ideas appreciated.

Cheers

Tony