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Look patent -- force-sensitive variable leverage

Title: "Actuating mechanism, in particular for brakes, especially bicycle brakes", European patent application; publication 0490196 A1, applicatio nnumber EP19910120600. Inventor Jean-Pierre Mercat. Filing date 1991/11/29.

This is a force-sensitive variable-leverage mechanism. It is the same principle as the Wilson Posiech brake [link], with minor changes.

Briefly, the cable pulls the two arms together moving the pads toward the rim by overcoming a weak spring and sliding one arm along horizontal rods. When the pads touch the rim, the force rises, causing the sliding arm to go "cockeyed" and jam. At the same time, on arm has a lever with pivot; cable tension overcomes a stronger spring and causes the pad to continue sequeezing the rim.

        

The effect is that the pads displace quickly (low leverage) but when they contact the rim, switch to high leverage.

Note the similarity to the Positech [link to image].

The application also suggests a stand-aline mechanism with input and output cables, which can be used with any cable brake.