The fuel line from the pre-filter to the fuel thermo DOES drain back on shutdown. This is normal.

Mine has a couple of very, very, very minute bubbles while running. So smalll and infrequent you are not sure if you saw them or not.

Is the car starting easy and continuing to run well? If so, I'd leave it alone for a while and see what develops.

If not:

Was the pre-filter oring replaced too? It has a separate part number.

Are all the fuel line white horseshoe clips loose and floopy?

If there is a leak in the line to the shutoff valve you would think air would be entering from above stream. I'd check the quality of your seal at the pre-filter again and maybe use a little silicone grease there. I've had trouble getting it sealed up before.

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Terry Allison
N. Calif. & Boca Chica, Panama

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