Thanks Rusty (on both counts:)!

That had occured to me and I did inspect it when I had it out for the gravity flow test.
But replacing it temporarily with a plug is definitely worth a try!

Finn

Russell Duffy wrote:

Removed flow sensor and filter and amount of bubbles appeared to be
significantly less. Put sensor back in, and bubbles reappeared,

 

Very baffling.  Here's my best 3 beer guess:

 

What sort of "drain point" do you have?  I'm assuming this is one of the many spring loaded valves that are commonly used to check for water in the low points of the system.  If so, and the spring is getting weak on the drain valve, I'd bet that's where you're sucking in air.  When you removed the flow sensor and filter, that reduced the restriction upstream from the drain point, and reduce the tendency to suck air in through the valve.   If this is one of those spring loaded valve, I'd start by changing it, or plugging it up temporarily. 

 

Good luck,

Rusty (giving Finn a break on the smart a** comments)