It is hard to determine where the leak is that causes a drain back. It could be from anywhere in the fuel system, above or below the level of where you see the drain back. Think of it this way, when fuel leaks out, no matter how small, it is displaced by air, which slowly works it's way to the highest point in the system. Your best bet is to fix any visible leaks first, one by one, till they no longer weeps, and see if that cures the drain back. If the air bubble/ drain back is still there, keep looking. A diesel fuel system should be totally dry, you should not see or smell any diesel.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked