brake warning light after rotor and pad change 91 100

[kolson@statestreet.com]

     On Saturday I changed the rear rotors and pads on the 91 100. Now when 
     the car is in running and in park and I step on the brake pedal, the 
     red brake warning light comes on momemtarily, and the pedal is spongy 
     through the first half of travel, then firms up and the light goes 
     off.
     
     Light does not come on while the car is in gear and moving, and the 
     brake function seems fine. I bled the rear calipers using the "pump 
     the pedal" method and with the car off, the pedal is firm and feels 
     ok.
     
     Car has ABS and I read that I should be using a power bleed system 
     from the master cylinder rather than the "pump the pedal" method.
     
     Anyone have any ideas on whether this is a bleed problem that can be 
     fixed with the power bleed method or something more serious?
     
     TIA,
     
     Keith