Could be the seals on the hollow bolt for the secondary fuel filter.
It will take a crush washer and an o-ring if it is an "01" head, or two o-rings if it is an "02" head.

Other possibilities where air incursion can occur are the primer pump, or the lift pump itself.
The lift pump has a small internal o-ring that does fail over time, and/or the internal check valve can fail.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022