Deisel911 - you answered the question precisely.

That Vacuum pump check valve screwed into the pump was all cleaned out hollow.

If the parts are lodged in the yellow area of the diagram, should I be
taking off the vacuum pump cover, removing that small piece there
to get these loose parts cleared out ?

(I presume that the small part in behind the cover is also another
one way valve which the pump uses to create vacuum ).

Are the small parts that came out of the metal screwed in check valve
a threat to the pump and its operation ?

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

I can only say that I have not read of loose Check Valve Parts damaging the Vacuum Pump.

As you said there is a Valve at the end of the empty space.
If you look in the Thread that Stretch posted I believe He has some pics of the Valves that are inside.

The Check Valve parts would have to break up into smaller parts in order to pass through the Valve.

However, it might be possible for some of the parts to block off the part of the Valve.
I don't know if that would cause any issue with that particular Valve or not.

If you decided to remove the Front Cover and pull the Valve and get the parts out (there is also an O-ring I think). when I took Mine apart I snapped off 3 of the little Cover Screws as they were corroded in.

I took out the rest of the screws by using Heat, Penetrating Oil, and a old style Manual Impact Wrench (the type you hit with a Hammer).

__________________
84 300D, 82 Volvo 244Gl Diesel

I still had low vacuum after replacing the one-way metal vacuum valve
screwed in to the vacuum pump on the 83 300SD.

I went after the debris from the previous one way valve that was open
straight through, that I thought must be there.

Learned a little. Will try and explain it. Did not bring pictures for exhibits.
But may add some later.

I fished for the parts through the opening where the one-way valve
is removed from the pump. This produced nothing.
So on to removing the pump front cover.

The vacuum pump front cover can be a real mess to get off for most people
because the screws are often rotted / seized and they have standard or slotted
heads. (I like the idea of replacing them with allen heads.

Because my car has been a California dry desert car its whole life,
the screws were able to come out without damage.

So here is where it gets good and could answer a lot about where the missing
vacuum goes.

There are three valves behind the vacuum pump cover, they appear identical to me.

They are positioned like this:

1

2 3

1 is the valve that pulls vacuum and leads to the brake booster.

2 and 3 are the exit valves that expel air on the up stroke and need to hold
closed on the stroke that creates vacuum.

In my case valve 2 was disintegrated and not doing its job.

In my opinion 2 and 3 must work for vacuum to work.
My next junk yard run will be me taking these out of the vacuum pumps as spares.

I removed the debris and small spring from the broken valve. Using the
remaining frame and put in position of valve 1.
Valve 1 was good so moved it to 2.

Keep in mind and watch carefully as valve 1 faces one direction poking
out of the pump and 2 and 3 are the opposite way and flush with the pump.
(sorry for no pictures).

So at this point I have valves 2 and 3 intact and working, valve 1 not working.
But because the one way screw in valve is there, and works, this makes up
for valve one having no guts.

After I put the front cover back on the pump (quite a challenge when
on the engine and vertical instead of on a bench laying flat).

All vacuum returned and brakes work and car shuts off.

I was almost ready to purchase a new vacuum pump.
Now I might just get a repair kit instead.
For sure I will start collecting these little valves from the junkyard.

Hope this post helps someone else who is missing vacuum.

By the way - there was no debris from the metal one way valve that screws in to the vacuum pump.
Is it possible that some cars came with straight through connectors ? I think so.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

Oh and how could I forget this gem the mechanic told me: after he "fixed" the shutting off, he said that he disconnected some vacuum lines and that made it able to turn off with the key. As I was driving home I noticed that the car was not shifting as smoothly as it had before. So, I went back to him and told him about it. And he said:"oh the vacuum doesn't have anything to do with the transmission!" I thought REALLY??? OMG!!