Another possible problem could be a clogged tank screen. It's a miserable job to check it though, so I would save it for a last resort. You might first blow compressed air from the front and have someone listen to hear if it bubbles up in the tank. I suppose that might rule out an obstruction in the supply line. You could do the same with the return line for that matter.

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine