Odds are that the leak is in someplace that you cannot easily find with the soap/water method.

Did you check the valves? I had a leak there and it was hard to find because I always had guages hooked up and that hid the leak from being detected.

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Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"