The larger the space, the more a vacuum system seems to dribble.
I have gallon small mouth jars that fit the same lids as standard quart jars. the gallon jars leak more when..
The temps change, the barametric pressure changes, the stand gets bumped or wiggled by the wind etc...
I think, weight and volume also have an effect. A lot more weight above, pushing down on the liquid opposing the vacuum.. so when they dribble they dribble a moment longer until the vacuum can overcome the added weight, as well as the speed of the syrup leaking out... Make sense??? My best guess....
I found Plastic gallon jars with standard small mouth lids.. I think it was from Pigeon Mountain... Those leaked even worse when the sun hit them.. or it went from 40 at night to 80 in the day. If the sun hit them it also warmed the plastic and let it flex more, which reduced the vacuum and let them leak..
These reasons are why I have built the four jar feeders to take 4 quart jars so I can still put a gallon of syrup overhead, without the issues caused by the larger space/volume.....
I use three or four holes in the lids, and they still dribble a little every once in a while.. I don't think you can ever get past a little bit of dribbling... but using this set up seems to create less of a mess.
I have also noticed on the large gallon jars.. If I want to give them a quart of syrup, it takes quite a lot of syrup running out before the vacuum builds enough to stop the flow.
Those are the things I have encountered... I like the idea a lot and am interested to hear how the testing goes. Please keep us updated on your findings and any modifications you make as you proceed!!!