To get working brightness key. try following
press Alt+f2 , execute
gksudo gedit /etc/default/grub
Change this line
to something like below
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
Open a terminal by Alt+ctrl +T, Execute
sudo update-grub and reboot . See if brightness keys are working.
If the above is not working set a fixed value for brightness. To do so,
Open a terminal by pressing Alt+ctrl+T.
ls /sys/class/backlight/*/brightness, copy the outputs to a separate text file
The possibility is there will be two paths,try the following for each path and replace accordingly
example paths will be like
Run following commands in terminal,
For the above path Get the maximum brightness:
Try a lower value to set the brightness, say output is 16 so I will try with half of it
echo 8 | sudo tee /sys/class/backlight/acpi_video0/brightness
If this works, make this happen in each login automatically by doing the following
sudo gedit /etc/rc.local
Enter this line just before
exit 0. It should look like
echo YOUR_VALUE_HERE > /sys/class/backlight/acpi_video0/brightness
Also you can try with xdotool