Benny,
I have remapped many of my keys, but brakes are the htc given keys on my system. Try Key C for left brakes, Key V for right brakes, and Space bar for both.
Hope this is helps, or is what you wanted. If by axis, you mean on your JS, why would you need brakes there? Stick with the keyboard, in my opinion.
Found as:
Options/controls/MapKey/Flight
Listed as:
Left Wheel Brake - C
Right Wheel Brake - V
Wheel Brakes - Space