E320cdi (W210) starting problems ....fuel shortage?

I recently bought a 51 plate 320CDI, which has done 200,000, but is a lovely car well looked after, 1 owner, fsh, etc, although it had had a starting problem just prior to me buying it which "seemed to have been fixed".
In the time I have had the car (3 months) it has now become apparent where the problem lies....The fuel system does not remain "charged" and for some reason the fuel filter housing emptys after a few days with no signs of leaks and this is when I have starting trouble, if I fill the housing with fuel, its starts! and runs ok for a few days but then it starts strugling to start again, so i have to fill housing again.
I assume this should stay full,(or should it?) Maybe it is dropping back? but I dont know why it emptys!
Prior to discovering this I was using Easy Start for a couple of weeks and it would start every time and runs superb and drives fine, plenty of power etc, but the fuel filter housing was still empty! so I can only assume that the fuel is being sucked almost directly across between the inlet and outlet pipes on the top of the housing, so this leads me to think that it is not a blockage in the pipe.
So I wonder is the fuel "pumped" prior to the filter? or is it "sucked" by high pressure pump at front of engine?
I have disconected fuel lines from top of housing (inlet and return) and found nothing comes out with the ignition on, or when cranking the engine.
I had it serviced recently (200,000) Mercedes Approved Repairer, and they couldnt diagnose any particular problem, but quickly told me that if I had further problems they would want it for a week, to run checks order respective parts and then check again over a couple of days, which is all very nice but I dont fancy the bill much!
Anyway hoped one of you guys may be able to shed some light on this for me as Im flumaxed?!
Thanks a lot, Ray.
( p.s when I say empty above, there is about 30mm depth of fuel in bottom of housing, Thanks again.)

It sounds as if you have a fuel/air leak that causes the fuel system to lose its prime.

Check the clear plastic fuel lines, they are sealed with a rubber oring. The orings deterioate with age and due to changes in fuel types (LSD vs ULSD) etc. Visible bubbles (no matter how small) would confirm this.

To re-prime you should manually hold key and crank starter 30 secs at a time. After 3 tries, let starter cool off for an hour.

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)