This was a fun lesson.
On September 27, 2010, Gil Dupre in Chattanooga, TN, USA. wrote:
"Car was running hotter than normal than normal 198 at neck top outlet of head. Water filled overflow bottle and it overflowed (MGB type). When I turn engine over with the radiator cap off water/ antifreeze burps out of the radiator. Even with cap on it burps out of over flow pipe. Compression check 150 on all cylinders. It even sloshes out with spark plugs removed. No water in oil, and no oil in water. Not losing water or oil. (Not gaining either). Timing is good spark plugs are brownish. Checked compression, 150 psi. Removed thermostat, 180, and it opens in hot water at 180. Removed new water pump impeller tight shaft good. Scratching head now. I thought I had blown head gasket until I checked compression, and cranked without spark plugs and it still burped. Any more ideas. Before I pull the head"?

"New hoses. Heater control valve is off. Never seen one burp like that. I have the radiator and water pump out now so I will check those hoses".

And then, -- TA_DA!
"I found problem. Last year when I replaced a bad pump I didn't get a gasket for it, so having made many gaskets in my life time I made one. The old one was destroyed so just made one for a snug fit and put some RTV on it and put it on. Worked fine for a year. On this last trip when temp was up slightly didn't worry because outside temp was 97 deg F. It was fine there and back. I noticed this problem one day before I was going to take on another trip.

"The suggestion of looking for collapsed hoses, I found a water pump gasket in my gasket drawer and compared to the one I made. Evidently I should have cut out a little more than I thought I should. Photo explains".

Next time make the gasket to fit the block not the pump. Lesson learned.