Thanks Bronze, that's good to know about the KFL/R91 tanks. I figured with the price tag they should be more durable. The strange thing is that all the cracks are in the thick middle part of the plastic section, not the thinner ends where the threading is. The tank still seems to be usable for now, but time will tell. I'm tempted to put the same juice in a vivi or aspire just to see what happens.
Yes, there are still a few quirks even among the finest of devices. You might want to post your question on this thread...
http://www.e-cigarette-forum.com/forum/rebuildable-atomizer-systems/436118-kayfun-lite.html
It's a Kayfun thread but trust me, we talk about the R91 and other similar devices. Plenty of folks with a variety of Kayfun-like devices on this thread. See if someone found a solution.