the KF has 5 o-rings that can be of concern, the 4 main ones on the
tank assembly and the one missed most often when rebuilding, the chimney oring that fits in the top cap. Depending on what version of KF you had purchased, the chimney oring can be either held captively in a groove or in the small cup milled into the top cap. The cup type method of capturing the oring lends itself to the oring staying on the chimney post and not sealing the
tank well, thus leaking....
if there is any type of grit fuzz etc on any of the tank orings, it creates an air path that also allows for leaks to develope. Next time you break the KF down for cleaning rebuilding check the condition of all orings for nicks and debris around the sealing surfaces.