Do you feel the shimmy by your feet? Does it go away/get worse/remain constant (intensity) under braking. I'm skeptical it would be the corrosion on the caliper piston before say a sticking caliper causing uneven pad wear/transfer to the rotor. How are u suspension wise in the rear? If your shimmy is coming through the seat bottom not the wheel or pedals, I'd start looking into the rear.

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