...but it's not. At least from a visual inspection. I stand the wheel up, rotate it, use a feeler to watch for front disc warpage, and I see nothing.

But when I actually ride the bike and apply front brake, particularly if it's a light amount, the brake pressure oscillates back and forth, almost to an unsafe amount.

Suggestions?

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"Because it's there"

I had the same problem as you can read in this thread here http://www.xt660.com/showthread.php?t=26643

I cleaned the bobbins, no difference.
I checked the caliper sliders - all nice and greased and not sticking.
I checked the wheel bearings - all ok.

I ended up buying some new discs and fitted them at the weekend. Bike feels so much better now.

I bought a pair of new oem front discs from Fowlers parts as they were only �95 for the pair including postage. Local Yamaha dealer was quoting �87 each!

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Bike: 2008 XT660Z Tenere with various bits and bobs added or subtracted