Stax and Stax of threads on this subject. Have a browse around and you'll find plenty of info with one or two copies of the exploded diagram. It's usually the Brake Rings that have gone, bought in pairs as it's two rings for one side. Normally just one side that's gone and approx £50 per side for the brake rings.
i had same prob i followed the download cleaned and re greased(i used bearing grease though) and they are working fine now:thumb2. as said above there is loads of threads on the subject if you do a search.