[drover]

I always bought new O rings from a Benz dealer, straight off the shelf for a few bob each ................... best thing for these engines is fit a new fuel filter and O rings , divert the return hose from rack straight to the tank no need for it to go to tank via filter, can cause air leaks, once thats all done fit a CAV filter at back of firewall at least level or higher than OE, then you can change the CAV filter with no stuffing around priming and the OE filter will last forever and you can change the CAV filter often for little cost ....
Run a cleaner thru the system which should clean the gunk from injectors, and sensors, never try to use the engine to fuel prime, a fast way to destroy the HP pump... a bleed back test on the injectors will show up if any are stuffed, if so take it to a Bosch diesel shop, saves a lot of stuffing around........