Cooling system pressure testing

[WinDancer]

My truck runs great and trouble free... EXCEPT when coming home up our two mile long rough driveway. About a mile up the temp climbs noticeably but still OK. By the time I get to out place, opend the gate, drive through, re lock the gate and pull into the driveway to park the truck is badly overheated. Right on the edge of blowing coolant.

I have been working to try to find the cause before it get seriously summer here.

Last night I pumped my cooling system to 15 pounds [engine completely cold] and left it for an hour. The pressure had dropped about a pound, and there was a single drop of water between the thermostat housing and the intake manifold. I slightly tightened both bolts and pumped it back to 15 pounds.
An hour later it had again dropped about a pound, but everything was completely dry.

When I loosened the tester the pressure was dropping and I went ahead and took it completely off- and there was a huge gush of air and water like happens when an engine is overheated [engine still completely cold].

How did pressurizing the system get the air into a position to bring water out along with the air?

Is this maybe a clue to what is happening with the heating up at long slow speed?

What [and how] can I check next?

Thanks,