If I get you correctly: changing screen brightness does not work. If so, I had the same problem and did the following for my (MINT 17.1) machine: I started using a script to directly change the brightness - not exactly elegant, but it does the trick:
search for you brighness control file:
manually change it as
echo $value > brightness, with the value being the desired brightness, e.g.
use a script do the same in future while unlocking the script in your
As script would look like
read -p "enter desired brightness value: " brightness
#check if value is a positive integer
if [ "$( tr -d [[:digit:]] <<< $brightness )" == "" ] &&\
#check value versus maximum value defined from the system side
[ $brightness -lt $(cat /sys/class/backlight/acpi_video0/max_brightness) ]
echo $brightness > /sys/class/backlight/acpi_video0/brightness
echo "invalid brighntess value"
Note: a value of
0 does not stand for a black screed (AFAIK), and on my system I actually got errors when trying to put invalid brightness values into the brightness file, so the
if seem to be optional.