Not really a "parking brake", it's more like a helper at a tow/recovery scene. The one's I've seen work by stepping on the brake pedal and then pulling the lever down which traps full air to all of the wheels...it's like having a person setting in the cab standing on the brake pedal while you work on the recovery. This wouldn't work as a parking brake as the air pressure might leak down over time allowing the truck to roll away!
Mike B
