Yeah I often want to do the same thing. I usually end up doing a combination of exposure, highlights, shadows and contrast. Remember, half (or thereabouts) of the contrast slider will push up the brighter tones. So using that, and then compensating the darkening of darker tones with the exposure and shadows slider, can sometimes produce an ok result. But yeah, I really wish the adjustment brushes had the same sliders as available globally. I don't know why they don't.