The most likely cause is the ABS/TRACS controller has failed. This is a common problem. Whereas you've just had the brakes worked on, however, you have to determine first whether the ABS system has been disturbed - sensors, wiring, etc. - in the course of the brake work. (TRACS uses the ABS system to retard front wheel spin in loss-of-traction conditions.)

What usually fails is the solder in the pc board under the ABS block in the engine compartment; a shop repair runs $350 and up. In a recent post to the brickboard, a correspondent indicated he was able to make the repair himself. (How hard can it be? Use a de-soldering tool to remove old solder, then flow the new stuff in.)

More technical information about this problem can be found at http://www.volvospeed.com/bay13.htm under Brakes and Wheels.

A recent thread here you may find useful is http://www.brickboard.com/AWD/index.htm?id=509090. This is a middle post, click up and down the thread to follow the traffic.
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(98 S70 T5SE misc mods - mostly lighting - and red calipers) (92 940GLE)