I'm unfamiliar with swcg (what does that stand for) and CYA?

One of the drain valves at the very top of the solar heater wasn't fully sealed. I managed to close that off yesterday, and for sure it was sucking in air from there. Perhaps it takes a few days for the bubbles to fully go away?

Is there an easy way to check lines for air leaks? None of the lines have water droplets but it doesn't mean they're not sucking in air?

Thanks for your help.