For well over a year, my KFLs have been leak proof during their normal daily use. I put them in my coat pockets without giving it a second thought. I lay them down sideways on my couch.
The only time one of them leaked was when I was driving back to Las Vegas from San Francisco. There was a long stretch where there was a major elevation change in the mountains. I reached over to my
mod cup holder and grabbed my KFL. It was leaking through the air hole and I mean it was LEAKING. Interesting note though, the GG Ithaka, which was sitting in the same holder, was just fine.
I'll try it again with the KFL. Next time, I'll turn it upside down when I encounter that particular stretch of road.