I have a 1990 Ford F250 7.3 liter diesel. There are no cooling system leaks - it hasn't gone down a drop of fluid in 35,000 miles, but the system simple will not pressurize, no matter how hot it gets. Cooling temp is normal, normally. Pulling 10,000 lbs uphill, it will get pretty warm but not overheat.

Any clues out there?

The first thing I'd do is put in a new t-stat. Ah heck, give it a flush while you're in there.

Check thermostat and radiator cap.