If there is air in the line at teh top of the pre-filter, the leak is at the pre-filter (most common) or preceding it (ie: fuel thermostat in head). I've had to use some silicone grease and also tweak the hold down metal clip to get it sealed again when replacing the pre-filter.

The oring you replaced was about ~3cm D x 3mm (~1.25" x 1/8") and was tucked up inside the underside of the old plastic pre-filter right? And, you ordered it separately for the pre-filter?

Since you are new, the little white horseshoe clips on the fuel lines should all be loose and floppy currently. THey are pushed tightly close (a ringer) to remove the lines. If pushed closed now they could be the source allowing air to enter.

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Terry Allison
N. Calif. & Boca Chica, Panama

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