Found this solution: I have an older 23" Cinema Display, a G5 Quad Core and am running Maverick. Come to find out the Maverick and Cinema Display are not compatible (per Apple), so no brightness controls available. Screen went irreversably dim after using Windows 7 (Boot Camp, partioned hard drive). Even on OSX. Nothing worked to fix problem. Fortunately, I also have a newer MacBook Pro. To get Cinema Display back up to desired brightness, I connected the Cinema Display to the MacBook Pro via Mini DisplayPort to DVI Adapter (and the USB plug), mirrored the two displays, and adjusted the brightness on Cinema Display through the MacBook Pro. Then unplugged from MacBook Pro, and repluggged in my G5. Started the G5 ... and brightness is back.
NOTE: Still cannot adjust brightness, so set it to your most desired level when re-establishing on other computer.