I don't personally know if this is possible but I think 2 members on separate occasions claimed the Delivery Valve Seals were a source of an Air Leak but did not leak Diesel Fuel out of them.

If that happens to be true in your case you could squirt some thick engine oil around the delivery valve seal area and that should seal it enough to block off any air leak.

I don't know much about the newer engines but if it has a Fuel Heater some have reported that it can be the source of an air leak.

The same if your fuel inlet hoses/lines seal with O-rings.

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84 300D, 82 Volvo 244Gl Diesel