The priming pump has a check valve, if working correctly, should not "suck".

If you can feel resistance in the lift pump plunger when you got most of the air out of the IP, the overflow valve is probably good enough to enable the engine to start

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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