On my old Compaq Contura 3/20 I can adjust the contrast, but there 's no effect using the brightness button. Is there something to do on this?
According to the Service Manual, first check if you're not using any power saving mode, as they limit brightness. Next check all cables, and if that won't work, replace the display (where the lighting tubes are included).
Chances are good that the CCFL-tubes are just worn out. So unless you intend to do some real low level work, you need to live with what's left.
Finding an 'oficial' replacement might be hard, as that includes the whole LCD module. On the other hand,the CFL tubes are ratehr standardized and still in production. So if you're able to comlete disasemble your Display and extract the CCFL tube, you could as well replace it. Maybe even with a modern LED stripe based replacement. Not classic but for sure extending battery life.
In any case, it won't be cheap. Expect something between 40 and 100 USD for the parts allone.