I saw a video on how to use the Adjustment Brush and brush on brightness gradually the more you brush over an area. I can't find it now. I am sure this must be simple but for the life of me I can't remember how to configure the Adjustment Brush to do this. It had to do with some configuration with these settings if I recall right.
Use the 'Flow' slider. Bringing it down in value will reduce the effect. You can keep brushing over the same area to increase the overall brightness (if that's the effect you have selected).
Density sets the maximum you can brush in; Flow sets the amount for each brush stroke. So if you set Density to 100 and Flow to 20, you will reach the maximum on the fifth stroke.