The stock bolt has an orifice to allow continuous bleeding of air in the secondary filter back to the tank. This orifice is plugged when converted to a 2 tank system to prevent the 2 fuels from contaminating each other.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked