Fittings from one end to another should be checked. We had a hairline crack in male end of the the solar panel diverter valve that caused all our air bubbles. It was not noticable except under close inspection. Just a little movement of the valve while inspecting caused it to almost fall apart - we caught it before that happened by itself, either by water pressure or someone bumping it, etc. What a disaster that would have been.

As soon as bubbles appear somewhere they don't belong, start checking everything before the minor crack becomes a failure, then a flood.