I also run with good success a hybrid system. On about 7-8 lines I have a gauge at the top tree and on 4 lines I have a gauge about 15-20' elevation above the mainline it empties into. That woods has a vacuum tank and I regulate the vacuum to 19". Then on the 2/3 of the bush where the 3/16 is practical because it has the necessary drop in elevation, on all I get 28-almost 29" on high barometric pressure days and on the 4 gauges lower down, it is either at that or less than 1 point less.
I'm with the General, your testing method is causing an error. It would take very little time, with 18" vacuum (on your old system) to empty the 3/16 line of sap, it had little or no sap left to give you any gain and with 3/16 being so small it did not get back up to even the vacuum the mainline had when you tested it. As said above, clamp the 3/16 line then pull the tap, or far better, just add a tee in the line where you want to get a reading and attach permanently a vacuum gauge. Those gauges will also help you identify when you have 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.