brake lever "clicking"

I have a 2003 XTR brake lever / shifter combo. There's a little threaded wheel in the brake lever
that can be turned to increase the leverage as the lever moves.

I have this adjusted to about the half-way point. When I pull the right-hand brake lever, the pin at
the end of the cable slides smoothly in the mechanism.

On the left--hand lever, however, the pin stays put until the brake lever has moved about 80% of the
way, then jumps suddenly, with a click that I can hear and feel. This doesn't give a gradual ramp-up
in leverage like it should.

I can't see what's causing this. I cleaned it and sprayed a little silicone lube in there, and
nothing changed.

Can someone suggest how to fix this?

Thanks