Air in diesel fuel lines C250

[Devallon]

My C250 starts well most of the time and then occasionally I'll have to crank until battery flat, recharge and go again before she will start. When like this, there are air bubbles in the fuel filter supply and return plastic pipes.
It looks like it is drawing air into the filter, so possibly from the pump ?
Plenty of fuel.
Anyone any ideas without substitution diagnosis, which can get very expensive !
Thanks

 

[bigtwin]

O rings on the fuel lines, cracked line, partially blocked fuel filter.

 

[Steve@Avantgarde]

Leak on the injector pots or the pressure solenoid on the side of the injector are the classic ones to go. You should see evidence of the diesel leaking should this be the case.