I found many people have this issue with older ladders. The biggest issue is older ladders use quite a different rung lock, so there's no replacement parts. The spring on my ladder is a coil spring, kind of like ones found in clocks. The end of the spring is folded over to make a clip that attaches to the side of the spring enclosure. The first pic shows this (the red line indicates where the clip broke on the other one). What I did to fix this is to use those fasteners you find on cars--called a spring nut. I clipped that over the remaining piece of the clip and that secured it to the side of the spring enclosure (see second pic). The spring nut is slim enough so it doesn't impede the functioning of the rung lock.