My first bike had brakes like that but I got one with calipers when I was about 10 (nearly 60 yrs ago). They were not exactly the same - they had post extensions from the ends of the stirrups that went through holes in pieces under the chainstays. These posts served the same function as the guides Reynolds mentioned. See if the stirrup is jamming in the guides. I think the return spring was a coilspring on the levers which worked the rods in compression.