If you have a leak it can usually be seen at fittings. Watch to locate the first place a fast bubble first shows in the line. However, sometimes it can vary like you are seeing but no leaks. The trees give off gases and those will look like air in the tubing. You will see a line of sap/air/sap/air and so forth all the way down. Higher up on the hill where there are fewer taps above it, the sap/air will move much slower, as you add taps going down hill the flow will start moving faster. Seeing sap and air exit the lateral at the tank is not necessarily a bad thing unless you find a leak.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.