I test ran the engine yesterday morning, after installing a freshly rebuilt water pump. The new pump seals worked great, no water leaks.
Last week I had the heat exchanger tested for leaks. The shop reported it did fine and there are no internal leaks.
All the hoses and hose clamps have all been double checked and are holding fine.
The new radiator cap is good.
I took the cap off while the engine was running and cooling water did not erupt out of the heat exchanger as it would if the head gasket were leaking combustion gases.
There is no evidence of external leaks anywhere.
There is no water in the oil.
There is no white steam in the engine exhaust.
In spite of this seeming water tightness, we still are losing about a pint of water from the cooling system every hour the engine runs. And I have no more parts to change.
I think that means it's time for some professional oversight. I have emailed the local diesel guru for help. He will be coming out soon.
In the meantime I will run the engine one more time this morning, bring it up to temperature, shut it down and remove the intake manifold and take a look at the intake valves to the cylinders. I had the manifold off the other day in order to replace the water pump, and noticed some moisture in the
passageway to a couple of the valves. I had spilled a little water over the inlets to the valves while removing the heat exchanger, but don't think any got by the duct tape I used to seal the openings. By bringing the engine up to operating temperature I can see for sure if water is leaking into the intake valves.
If water is present I may have a problem.