I update windows 8.1 my Vaio YB series laptop with AMD E350 APU. But brigtness is at maximum and it doesn't change. AMD graphic driver seemed to be source of the problem. Because screen brigtness is working perfectly after uninstalled AMD graphic driver. (using generic windows driver - Microsoft basic display adapter).
I updated all drivers on the official sony website, and i have tried Amd catalayst 13.9 and 13.11 Beta drivers. But problem still persisted.