Backing up with surge brakes can be tricky. You are pushing against the brake master cylinder
CaptVR here, Surge breaks, depending on the manufacturer have either a pin, or a lever like on a trailer winch, that locks the movable slide from sliding. This does not allow ram from setting breaks, this allows backing, and maneuvering be it a camper or a boat. Had a friend that use to bitch about his breaks binding up when backing, especially up hill, complained for three month. I told him he had a lock out, he told me he did not. I took him out to his trailer, now when I see him, he puts his hand over his face and says color me red. He said he seen it but thought it was part of the surge breaks, I looked at him and said "IT IS"
Everyone boat safe. Capt. Vince Rakstis, Ret.MS St. Petersburg, Fl.