I'm trying to replace the primer pump on an 84 300sd. I've tried several different ones (I have 3 of them). I've tried 1 copper washer and 2 copper washers. I've fully tightened them. Each time there is a slow drip from where the primer screws into the holder on the side of the engine. I've replaced primer pumps before and never had a problem. What should I try?

 

Those drips are common on the OM617. Few things to try:

First, use one new copper washer, not two. They crush to seal. Two washers prevents that.

Second, clean both mating surfaces really well. Old copper residue stops the seal.

Third, torque sequence matters. Tighten to 30Nm, loosen, tighten to 30Nm again, loosen, then final torque to 35Nm. This lets the washer seat properly.

If it still drips, check the holder on the engine side. Cracks or pitting there mean you need a new fitting, not just a pump.