Mafac squeal:
1. A ride in the wet usually beds the pads nicely. The squeal then stopped on all my Mafac-equipped bikes.
2. Matthauser cantilever pads work great and squeal less, especially after 1. has been performed. (Available at http://www.rivendellbicycles.com)
3. Brazed-on pivots seem to eliminate some flex, as those bikes squeal even less. (But of course, that option should be pursued only when ordering a new frame... not worth doing just to cure the squeal.)
4. The pivots on Mafac brakes wear. If there is a lot of wobble in one arm, that may contribute to the squeal. More toe-in would be an easy solution (preferably by sanding the brake pad rather than bending the arm.) Or new bushings. Anybody interested in making some? (It's an easy-to-make bronze bushing, anybody with a lathe could turn out 200 in an afternoon. I'll buy 52, if reasonably priced.)
With 4 Mafac-equipped bikes, and very little squeal, I have not yet toed in a caliper.
Jan Heine, Seattle