I experienced something a bit unnerving yesterday on a
bike I have been riding a couple years. The bike is a
nifty steel sport frame. The brakes are Ultegra 49 mm
reach with Kool Stop pads. On descents I was waiting a
bit longer before braking at turns, using more braking
force, but still using progressive force, rather than
slamming them. At a later point in the brake
application, the braking force suddenly increased
sharply.

Check both the rim and pads for signs of unusual wear; specifically, if the
rim is significantly worn, you can end up with a lip near the top and
bottom. As the brake pads wear, pressure is at first applied to the very
small surface area of the lip, after which it drops down into the main
section... which can suddenly increase the braking forces.

--Mike Jacoubowsky
Chain Reaction Bicycles
www.ChainReaction.com
Redwood City & Los Altos, CA USA