I am not up on 3/16", but am interest and keep reading more in case I have an area where I can benefit from it and I can pass along information to others.

I don't believe the mainline should be involved with helping with vacuum in these cases like the 3/16" does itself. My concerns with smaller mainline is freezing of the sap....it does not take much of a sag or zig or zag to create an area susceptible to freezing. We found this to be true when we used 1/2" line or did not pull our 3/4" tight enough. We are at the cross over point for 3/4" to 1" now. We figure the extra diameter would help with vac transfer (we use vacuum) and reduce full pipe freezing. When we ran new mainlines we stepped up to 1" and then dropped our 1/2" on the laterals and are running 1" on the larger runs and 3/4" on the shorter.

If you have any mainline run now sight along it. Look at the smallest drop...it would not take much for that drop (droop) to allow full pipe freeze. Depending on sunlight you could loose most of the day.

Again I don't believe the intent with 3/16" line is to have the mainline help establish or keep a natural vacuum...

Mike