When I worked in a Fuel Injection Shop during the rebuild of Fuel Injection Pumps the Delivery Valves were inspected visually.
For small issues they were lapped in a similar fashion to lapping a Valve with 1600 grit grinding compound.
Other types of damage or wear called for replacing of the Delivery Valve.

With clean Diesel Fuel there is no much to cause Delivery Valves to stick or gunk up.
Water sitting in the Delivery Valve area overnight can sometimes cause them to stick as can old dried Diesel Fuel if the IP has sat and not been used for a long time.

Sometimes the Delivery Valve Spring fatigue and break and I have also seen them rusted through.

Anything gritty going through the Fuel Injection Pump is more likely to damage the Elements/Plungers and Barrels than the Delivery Valve and wear on the Elements causes eratic Fuel Delivery.

When an inline type Fuel Injection Pump is rebuilt they indeed use a degree (Motorcycle shops used to sell them) wheel on the end of the camshaft and each of the Elements is Drip/Flow Timed and Begin of Injecion is set for each Element.

Another area that causes erratic Fuel delivery is wear between the Injector Spray Nozzle wall and the Nozzle Pintel. This is another area like the Elements were grit will cause lengthwise score marks on the precision surfaces.
Wear in this area causes more Fuel to leak buy and go out of the Fuel Return Lines instead of being Injected into the Engine.

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84 300D, 82 Volvo 244Gl Diesel

I agree with your post needless to say. My concern has been slowly increasing because of the alternative fuel practice. Plus the seemingly not changing the fuel filters generally unless they choke off or perhaps a person sees a long trip ahead of them. Some dirt etc will migrate through that pretty well loaded filter and process through the pump. Especially when it is being pushed so hard through a restricted filter.

Since you are also a very observent member and well versed on site. I know my suggestions at this point in that area are not well considered. I feel stronger over time that some of my opinions have more weight to them than they had some time ago.

Funny how I still question the number one rod failure thing still but less and less as time moves. Now to me at least some evidence is surfacing. Really felt out on a limb with that one.

Basically in my opinion it is false economy in more ways than the majority think to have too much restriction in the fuel supply system to the base of the injection pump. So much so a person should really have a gauge to watch for when that pressure starts to decrease beyond a certain point. Then rectify it. The gauge is also a very useful addition for troubleshooting in the system that causes the most grief on these old diesel engines. It will in many cases provide an early warning system that there is a problem somewhere.

I think it has to be a given now that all these engines and the fuel injection pump function best at about 19 pounds base pressure. It also looks like the manufacturer is in general agreement.

What has all this to do with the delivery valves? Reciently there have been some reports of finding a lot of solid particles on the seats and valves on old pumps running only diesel fuel. I figure it has to be getting by the seldom changed secondary fuel filter. Logically if they are appearing on some cars they will be on others as well.

Then there are all the threads where the poster has done all the so called normal things and the problem is continuing. Especially the rough idle incidences.

At that point I can see little harm in testing the delivery valves for their sealing qualities. Using things like the milli volt system will locate the suspect injection pump element. It is still cheaper for us to spend some time instead of money to clear or condem things of having possible problems. As long as a test is cheap and possibly indicated for a suspect condition.

You want to systematically condem or clear portions of the various systems. Can we change a defective delivery seat that will not test properly ourselves? That is one that is not responding to cleaning? I know we can change the moving valve portion if defective ourselves.

Most these pumps are just so old now and getting older by the day. I personally like an engine that meets my expectations. My four mercedes diesels engines in general run as perfectly as the type can generally speaking. Just a matter of good coincidence and I can take no credit for this.

Too bad my two carb fueled gas mercedes cars are not simular. I have owned mercedes cars with perfect functioning zenith carbs but not at this point in time. Those carbs and systems need work.