I'm open to suggestions on this one. Cantilever front brakes, Kool-stop black shoes. Under front wheel braking there is a rhythmic <thunk> — one per wheel revolution. This started suddenly going down a steep hill. I figured a front ball bearing had gone and replaced them. But the balls and races and cones all looked good. And the <thunk> is still there.

When you look down on the fork you can see that it is being forced backward briefly, like the brake had suddenly gone to full lock then releases within ~5° of revolution. I can't feel a pulse in the brake lever but it's hard to tell since the <thunk> shakes the whole frame. Intensity is proportional to load. Can't feel it on the stand. Spun the wheel: the rim and tire look fine. I don't see anything like a bulge or dent on the brake surface.

I suspect the <thunk> itself is a result of too much play in the headset, but the cause has got to be in the wheel and brake interaction.

Ideas?