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.