[Dnick]

Originally Posted by Webwader

Drum surge brakes need no electrical connection but disc surge brakes do. Because disc brakes work equally well in either direction and backing up, especially up a slope, activates the brakes, a solenoid is required lock out the brakes in reverse. The reverse wire activates the solenoid. Without the lockout, disc brakes can lock the wheels backing up.

Thanks Webwader - I just learned something.