[Epixam]

Im not sure what exactly the problem is, as i have a T420 and am having a similar issue. A temporary solution for the problem would be installing "xbacklight"

Its obviously not an end all solution, but i have been trying to find out what the problem is for some time and to no avail, and have been using this solution for the time being

Ex:

xbacklight -set 80

that will set the backlight to 80 pct