im thinking instead of having a button to automatically be one 100 percent on brake , Just have the brakes set on normal. it should Be registered as 100 percent if assigned on a sip/ Puff. And your second version of breaking can be of increment value If you so choose.
left_2, normal, mp_center_sip
left_2, increment_value 50, lip
I believe the center sip should override increment value As one way to turn off left_2. Alongside with force offs