First, crack them open and lookd for a "sub brightness" control, or simply try adjusting up the SCREEN control on the back of the flyback transformer. Both should increase the range the "user" controls can give you. You can also play with the RGB drive and cutoff controls on the chassis/neckboard, however once you start with these, rebalancing them can be a pain, but doable.
Usually when PC monitors go dark, it's more than just caps, as those usually affect deflection/geometry and not the pure drive intensity... it's usually the tube itself or other more significant components.
You are welcome to try to recap it and see if it helps, but since you can get new PC CRT monitors relatively inexpensively, it usually isn't cost effective. Though 19" is about the level you need to make the decision at... 17 and smaller = toss.