if the sensor is open circit it sometimes thinks the wheel doesn't need as much stopping power
i can't really explain it but i'm sure someone else will come along soon enough to tell you

when i did my abs sensor i had actually chewed through the sensor when the wheel bearing went and i had the same spongey pedal until it pumped up
change the sensor or even unclip it and see if it makes a difference

one thing was that none of the lights came on but if it's the handbrake light you shouldn't realy be driving it