Join Date: Dec 2009
Location: Melbourne, Oz
Bikes: copy/paste links: http://velospace.org/node/36949 http://velospace.org/node/47746 http://velospace.org/node/47747
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Wow, that thing's old hat.
As for ideas, examine it till you get some of your own. The longer you look at it, the more likely it is you'll understand exactly how it should work and why it's not.
I've never worked on such brakes and I doubt many around here have, so you'd prolly be lucky to get any specific tips without detailed pics.
That exploded diagram gives a rough idea of the principle of its operation but does stuff-all to tell us what your problem might be.
If it was my problem and I couldn't work it out, I'd completely disassemble, clean and inspect it. Doubtless I'd discover the problem then or upon reassembly.
I'm guessing either something's bent, too worn or gummed up with crud.