Injection pump questions (general)

I have three trucks that appear to have IP issues. Getting someone to look at them who know what they are doing is impossible. I HAVE to have these trucks running. I know IPs run on pixy dust and unicorn poop. So I need to take the mystery out of them. I have looked on line but could not find any good information. Is there a reliable source or a good downloadable manual I can access? I want to learn how they work and how to rebuild/repair them.
I have very limited access to the internet right now. So please bear with me if I cannot respond right away.

Your question is too general. All fuel injection pumps, as they are typically referred, my not be "injection" pumps, by definition. Common distributor type pumps (Stanadyne, Roosamaster) all work about the same, within their respective technologies. Mechanical are different than electronic, although their basic fuel delivery mechanics (axial piston) is at least similar. Non-distributor types (such as common rail), are very different, as they do not "distribute" fuel, they only supply the specific fuel flow and pressure to something else that determines injection event timing, volume and pressure. HEUI (Hydraulic Electric/Electronic Unit Injectors), and their system, are in a class of their own, as it uses two hydraulic systems, plus electrical/electronic systems to operate (engine oil system, fuel supply system, vehicle computer system, and vehicle electrical harness system). Powerstroke/Navistar T444E 7.3L engines use this system, for example. Add to that, the fuel injector components and technologies, and it gets even more scattered from there. Unless you intend to study and become a general Diesel fuel system technician, limit your questions to separate and specific topics related to the exact system you are asking about. There is no single or simple answer to the question you asked.

I planned on asking questions about the specific IPs I am dealing with with when I am home and able to work on them. Right now I am just looking for a general understanding so I can get familiar before diving into them. I do know the pumps I have are mechanical at least. I was worried that this might be a more complicated subject matter than I originally thought.

They aren't complicated, independently. Lump them together, and it gets complicated. Most of the technologies don't cross over. Short of trade-school level publications (which are almost always only available to enrolled students), it isn't likely you'll find anything that covers them all. On the bright side, you have Google....

....Forgot to mention inline distributor pumps. That's a whole world within itself, and the most common among older platforms and vehicles. Tractors, Bosch-equipped vehicles (Mercedes, VW, Audi, BMW, etc.), and many HD engines, such as Caterpillar.

Then there are racks (Detroit), cam'ers (Cummins), and a long list of others.

Lonesome, to narrow things down a little, are you working on your trucks and are they old military 6.5s? And are they DB2 pumps?

Two of them are multi fuel engines and one is on a 6BT. i have not even looked at the 6.2 in my CUCV yet.