Click on the adjustment brush tool, note the settings of the sliders, then zero all of the sliders.
Start an adjustment brush, set one of the sliders to some value.
Close the adjustment brush by again clicking on the tool.
I believe that the next time you click on the adjustment brush tool, all of the sliders will be zero. I also believe that, if in the first step you set one or more of the sliders to some value, then when you start a new brush, the sliders will have the values you set the last time you set the sliders without having an active brush.
This is the behavior of my adjustment brush settings in LR4.2 on a Windows 7 64-bit system.