No such toggle.
You could reset the brush and then undo the reset.
I think I know what you mean but if you do Reset it resets ALL the brushes instead of just the one. The OP just wants to toggle one brush.
To toggle one brush.
Click on the brush you want to toggle.
Double click the word "Effect:" int the upper righthand of the slider box, This resets the sliders to zero
To reverse, you can click undo
To see the difference many times, just click back and forth in history to see the changes before you do the undo.
Thanks - that was similar to how I had been doing - deleting the active mask then hitting undo -- your method is a little quicker/easier. Using the undo/redo history is just a bit clunky and limiting so I was hoping there was a better way.
It would be a nice future enhancement to be able to quickly toggle each mask separately (not sure the best place to submit feature requests - but I will search around)