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I've noticed similar things with my ABS, just the way you're describing it. We own the same model, even the colours are the same.
This is not an ABS fault. When you slightly brake on ripples the braking power is over powered by the friction on top of the ripples, so your wheels are turning.
Between ripples the slight braking power momentarily blocks the wheel while the wheel is in the air, however the ABS does not know whether the wheel stopped on the surface or in the air.  The ABS knows that the wheel is blocked and it has to release it.  At least this is how I can explain the phenomena.
Bob Silas
  ----- Original Message ----- 
  From: John Dancoe 
  To: oilheads@xxxxxxxxx ; John Dancoe 
  Sent: Friday, November 11, 2005 6:29 PM
  Subject: RE: ABS

  I know I'm going to regret chiming in on this subject again...but here goes

  Two things I noted in the Kneebone article.

  1. Of the various surface/obstacle conditions tested, rough rippled pavement
  was not among them. It was precisely this condition which, on multiple
  occasions and not just in Michigan, caused anomalous ABS behaviour on my bike
  that flat scared the hell out of me. I was, and remain, convinced that those
  specific, particular situations would have been utterly non-memorable if the
  ABS had not activated. Conversely, I do not have any "ABS saved my butt"
  stories (though I'm sure many riders do).

  The roads in Michigan have not gotten any better since I removed the ABS.

  And I've not had a single terrifying experience while riding since then,

  2. "Although ABS bikes have several more feet of plumbing, which could
  theoretically make for mushy or less reactive brakes, except for our Racer
  (who insisted all the bikes needed braided steel brake lines), our testers
  felt the ABS bikes equaled the feel of the stock machines."

  Gotta agree with Racer: my RS brakes always had a disgustingly mushy feel,
  from day one and no matter how often the fluid was changed, until I removed
  the ABS. Since then, with braided lines up front...oooooooh what a nice, firm
  feel. Night and day difference. I savor it every time I ride, this morning's
  commute included.

  Now, having said all that (and hoping here's a bit of fire-prevention):

  A. The article--not to mention persuasive arguments from the likes of listers
  like Steve M.--is pretty convincing. Not quite enough to make me want to
  re-install my ABS, but enough that I'll certainly consider an ABS-equipped
  bike when the time comes to replace my current mount; but you can bet I'll
  hunt down some seriously screwed-up pavement for my test ride.

  And yes, yes, I know I'm making the utterly unwarranted assumption that I'll
  somehow survive without ABS until then.

  B. It's my understanding that BMW ABS has evolved at least one generation
  since the ABSII which came on my early '93 RS. Quite possibly a unit with a
  faster modulation rate would not be affected by rough pavement the way mine
  was. Maybe I'd like the Honda or Yamaha ABS a lot better. I'll find out,
  because I don't plan to automatically replace my RS with another BMW.

  C. Maybe my particular ABS unit was defective all along. I know for a fact it
  was set up wrong initially: it was way too trigger-happy--but I didn't know
  any better--until the first time I replaced the front tire which (I theorize)
  resulted in the sensor gap finally getting set correctly. But the realization
  that the system had all that time been seriously malfunctional, and had
  unquestionably compromised braking effectiveness, certainly left a sour taste
  that never went away.

  John D
  '93 R1100RS x-ABS x-Bing x-teal seat