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Put the binding back together (everything but the half-moon steel plate) lift the brake up as high as it will go. Place spring with the two metal prongs down and the one rounded on up touching the bottom of the brake pad. Push it in with hands till snug... Get a big flathead screwdriver and hammer and go to town. (don't be too rough though)
Trust me just use a hammer and flathead screw driver, (even a chisel will work) and just pound it in there. After it gets lodged between the two brake arms your basically done and only need to tap it in the rest of the way.