I think GM installs gremlins in these trucks. Strange things like loosing prime and finally maintaining the prime happen sometimes.
It seems very strange, but I see this in the power plants a lot too, things are going to crap, and then suddenly the problem(s) goes away.
My truck had/has a PS fluid leak. I think I pinpointed the leak. I was adding fluid every few days. I checked it last night, and the level has stayed right about the same, if any losses. The leak may have just slowed dramatically.
If it keeps happening, you may have a crack in the housing or small nick in the o-ring. These things can sometimes change with temperature.