1998 citroen relay 2.5 non turbo d bad starting?

must have a problem with air in the supply side of the fuel system. wont start unless i hand prime the filter. i have changed the filter element, primer head,and the spill pipes from the injectors and the return to the pump also replaced the glow plugs initially thinking that was the problem but that made no difference at all. after priming the system it will eventually start but after a few seconds the engine revs drop right off presumably because air is still present in the line then after this time it will tick over ok and you can drive. if you dont give it the time it needs and drive off straight after starting it will lose all power and stall. the only part i havent replaced are the fuel lines from the tank to the filter if any one can give some pointers or have had this same problem and can help it would be much appreciated, by the way there is no evidence of fuel leakage PLEASE HELP THANKS!?!?!?!?!?!