From the looks of a cutaway image of that valve, you have two potential trouble spots in it. The two o-rings, and the assembly nut. The smaller o-ring is for the stem, and could be the only problem. The end connector has the larger o-ring, and would be less likely to fail, as it is a non moving part. The assembly nut could have vibrated slightly loose, allowing the end assembly to slip just enough to allow air in. When you ran water over it, that gave the pump just enough "pull" to bring the end assembly back into place.
Check the nut, and make sure it is tight, and that the end assembly cannot move in any direction.