It's a slot-head screw on the top of the pump, at the rear. Exact position depends on model.

You can also pump forever and loosen the return line, but that will not purge the air out of the fuel gallery where it goes into the sleeve/plunger sets.

It is usually fairly obvious -- a slot-head screw that doesn't hold anything down.

A buddy of mine has bought five or six W115's over the years for a couple hundred bucks, usually only needing new fuel lines to work properly!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!