backlight does not dim on AC adapter unplug

[dg]

I expect my laptop backlight to dim to when I unplug the AC adapter.

This never happens.

Laptop: Asus N56VJ
Kernel: 3.9.0-rc6-custom+ (running Aaron Lu git branch, which fixes fn-F5 and fn-F6 screen brightness hotkeys)

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: gnome-power-manager 3.6.0-1ubuntu1
Uname: Linux 3.9.0-rc6-custom+ x86_64
ApportVersion: 2.9.2-0ubuntu8
Architecture: amd64
Date: Sat May 25 17:02:17 2013
InstallationDate: Installed on 2013-05-15 (10 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
MarkForUpload: True
SourcePackage: gnome-power-manager
UpgradeStatus: No upgrade log present (probably fresh install)

[chreko (chreko)]

Add the following script to /etc/pm/power.d
----------
#!/bin/bash

case $1 in
    true)
    echo "Enable screen power saving"
    echo 5 > /sys/class/backlight/acpi_video0/device/backlight/acpi_video0/brightness
    ;;
    false)
    echo "Disable screen power saving"
    echo 10 > /sys/class/backlight/acpi_video0/device/backlight/acpi_video0/brightness
    ;;
esac