Two questions:
1 - Are you filling the tanks with the top cap off? What is your current method of filling the tanks?
2 - What is your vaping techique? Are you a hard chain vapor?
Filling a tank with the top cap off is a risky situation. Once you add the top cap, the increased sudden pressure within the tank causes too much juice to enter the cartomizer and floods it. It has no where else to go but into the air tube and out the connector, and once the normal fluid barrier has been breached, the carto is doomed to continue leaking in this manner.
Puffing too hard on a cartotank will cause too much juice to enter the carto, also. A gentle, sustained, deliberate draw of 3 - 6 seconds works best. Think sucking on a straw in a thick milkshake.
Since this is happening in a variety of tanks and different brands of cartomizers, the problem is user error. Click on the link below my avatar under blog entries. The second article is on cartotank setup, with step by step instructions on how I personally set up my tanks, including a couple of videos that demonstrate these techniques.