If the Plungers are not going up and down then the Plungers are stuck upwards.

To add thumbnail type pics transefer your Puctire to your Computer. You need to type what ever you want to say in your Post and then go down and find the advanced button. If you click on that an scroll down you will see manage attachements.

Click on manage attachments you will see a list of typs of items that can be attached as a thumbnail and also the size allowed. Assuming you pic is small enough you go to the top where it has Browse.
Click on Browse and go to the place on the Computer where you stored your picture and select that and click on open.

After that the loction of the pic should show in the Browse Box. Find upload and click it.
When the Pic is up loaded you will be ask to close that whole Box. Close it and save the Post.
And that should be it.

If your picture is too large go to Microsoft Paint and open your pic up in paint. Search the top Menue till you find Resize and Skew. Select Pixels and that will tell you what size your pic is. Buy changing the size in the Pixel Box you will change the size of your image. I think 800x600 pixles is what is allowed for a JPG image or a PNG image.

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84 300D, 82 Volvo 244Gl Diesel

Is this valve 2 pieces? The one that sits inside the hole in the injection pump? Should the center piece be separate from the body?

The valve is two pieces - the round barrel and the plunger. Mine were stuck together.

I actually have exactly the same issue. Did this pump ever loosen up?
I've got a 71 W115 with a 75 240D engine and it's sat for 10 years. Lift pump was stuck and all 4 plungers were stuck. Still not getting any fuel out of the injection pump. The rack moves just as much as on this car.
I started a thread over here:
1971 240D Newly aquired. Injection pump not building pressure. Car sat for years. - Mercedes-Benz Forum

The elements must be stuck as well. I think they should rotate as the control rod(rack) moves back and forth and control the amount of fuel. The movement we are seeing I think is just free "lash".
Any tricks to free those elements up without messing up the calibration?