I have a 2000 S70 also, and what I think you guys are talking about is the EBD(you can see the light on the dash, and read about this in your owners manual).
What EBD(electronic brakeforce distribution)does is as you apply the brake....the car will automaticly send more of the brake force to the rear wheels than normal. The fact that you both said your cars do it over a uneven road surface.....is exactly how it should work...the ABS knows the front wheels are not "grabing" like they should, so the EBD unit kicks in and sends some more pressure to the rear calipers...in what feels to me like "pulses".
Just a thought....but check your owners manuals for a better explaination.
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