Air entering the prefilter has to come from a bad seal at the other end of that small line (connects to the fuel thermo/heater) or from the rubber fuel lines leading from there to the fuel tank.

Why did you change the DV's, were they leaking fuel?

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Terry Allison
N. Calif. & Boca Chica, Panama

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