Installed Slackware 14.0 on my older Gateway CX2756 and it works great right out of the box and now its time to make all sorts of tweaks and I ran into an interesting issue. The function keys work and I can use them to control volume and brightness, but the brightness gets wonky. Its like it doesn't know what the min brightness is so when I use the brightness down key, starting from max, it will go down till it gets to a point where if I take it down one more step, it gets brighter. Like it hits bottom but then bounces back up.

I'm using XFCE and it shows the brightness scale when I change the brightness level. When I get to the point right before the weirdness happens, the level meter has one more segment left, then I try to turn it down again, the level is shown at zero but the actual brightness jumps up. I can keep hitting the brightness down key but no change. I use brightness up and boom, the brightness level meter shows up with it just a step below max. Not sure where to even look to start investigating this and I hope I explained this clearly enough.

Any ideas?

Try monitoring the output of
/sys/class/backlight/acpi_video0/brightness and repeat your experiments.

Looks like the step value for each up or down is off? Though when I reach zero, its not actually(as seen by my eyes) at min brightness.
So starting from max, brightness=8 > 6 > 4 > 2(lowest visual brightness achieved) > 0
Then going back up 0 > 5 > 7 > 8.

Thanks for the hints, did some more digging and found /sys/class/blacklight also includes intel_backlight. I found I could set the following kernel parameters "acpi_osi=Linux acpi_backlight=vendor" which gave me full control of the brightness(via the xfce brightness slider), but renders the keyboard controls useless. So I'm guessing Linux will use either the acpi control or the vendor control but can't use both?

I also noticed the following in /var/log/messages:
logger: ACPI group video / action DD02 is not defined

and DD02 looks to be the keyboard keys used for brightness control
acpi_listen:
video DD02 00000086 00000000
video DD02 00000087 00000000

When I started at this I grabbed a fedora acpi and a debian one. Both had loads of scripts. I hacked one to suit my hp at the time and made it 2, (one for up, one for down). They were installed in /etc/acpi and called from /etc/acpi/events. I am no scripter, but I managed. Try it for your box. I can only give you for mine.

I looked that up and think I got it mostly working, but seems like XFCE doesn't want to let go. I can manually use xbacklight to change the brightness and use that in the acpi handler script for when either brightness up or down is pressed, but no luck. Setting acpid to log everything shows its processing everything right, as far as I can tell, but seems like XFCE is still doing something. Is there a way to have XFCE ignore these keys? Or change what XFCE does when they are pressed?

xfce has a power manager which expects to be the only player in the field, and duplicated some of the BIOS stuff. Itś easy, for instance, to get yourself a ouble hibernate'every time you hibernate.
The bios and xfce raced to hibernate, whichever lost trapped the signal and used it as soon as you restored. I also have keyboard issues on a box which show in brightness controls (works from an extermal kb)