I don't have a clue on solving that on software, however some time ago i had the same problem, simply opened the monitor's case and there were 3 internal regulators, brighness, focus and something else i can't remember(they had names on them, i think brightness was "scr"), if u fell comfortable give it a shot, just beware of high voltages.
These are on the eletronic board and on the black thing attached to it, don't mess with the ones on the tube, those are for color and stuff
Of course i am not responsible for any damages
I recommed u use a isolated screwdriver for this task
(yeah this is not much of linux stuff but it's all i kown about this)