Please try starting the attached script in a terminal "./gpmhelper.py", and do a single press on the brightness-down button. Please attach the generated output. (Your brightness will seriously misbehave while this script is running, but that is not interesting, just the output is). You exit pressing ctrl+c.
After doing this test please try starting the script again. Please press your brightness-down button down and hold it down; wait two seconds and release it again. Once more attach the output.
Please try starting the attached script in a terminal "./gpmhelper.py", and do a single press on the brightness-down button. Please attach the generated output. (Your brightness will seriously misbehave while this script is running, but that is not interesting, just the output is). You exit pressing ctrl+c.
After doing this test please try starting the script again. Please press your brightness-down button down and hold it down; wait two seconds and release it again. Once more attach the output.
Thanks again for your feedback.