I may use mine on an M105 frame/axle...not sure yet....more likely will be extending the tongue and using this one with the hitch modified for my 2 5/16" ball (can change to lunette ring). While I trust "free backing" in general, I have not studied it to understand it's workings. I want to have the possibility of making the brake unit rigid, mechanically locked.
While going through the surge brakes available at etrailer, I noticed no backup lockout, mechanical backup (which I believe is a pin) and electric. I didn't know the electric was acting on the brake line, closing a valve. Sure, I see how connecting to back-up lights would actuate the electric backup.