problem to set volume level with push button

hi, i like that when my volume is not equal to zero pushing a button set volume to 0 and that when my volume is not equal to 0 when I push the same button my volume is set to 0db so 0.85 in the reaction but my script only work to set volume to 0 and not in the other way, anyone can help me with this please?
I attached a simplified script with only that function,
thanks