There is a recent article by Tim Wilmont, UV Proctor Maple Research Center in the Maple Syrup Digest. It explains in detail their findings for using 3/16" tubing for gravity systems. The performance looks to be excellent. Sap gallons per tap hole were equivalent with a vacuum pump system. A decent amount of fall is needed for this to work. There is also information as to where to find the fittings and tubing.

I have about 40 sugars I hang buckets on, just over a hill. I don't want to try hook them into my vacuum system because of cost and how far they are away. If I add the reds around them I will be at 75 taps. I can put a tank at the bottom of the hill and I should have about 30' of fall for many of the taps. I already drive to the bottom of the hill with my BAT mobile to collect the buckets. A little more road improvement and I should be good. I wasn't going to do any expansion this year but this could be a fun little project