Are you priming the coil by dripping
juice into the coil from the top? The only time the Subtank Mini has leaked on me through the air control ring was when I over primed the coil by dripping too much juice into the top of the coil. It dripped through the bottom of the coil and leaked out through the air openings. Since then I prime the coils predominantly by putting a drop of juice onto each wicking hole, wait for it to soak in, and then repeat about 4 or 5 more times. I will put just two drops down the top of the coil with the coil tilted to one side, so the drop soaks the cotton on the side and not in the dead center of the coil. Then screw the coil into the tank, let it sit for 5 minutes and take a few unpowered draws with my fingers covering the air intakes.
Haven't leaked since.