had same problem a few times after removing the sensors on the front and re installing,abs was cutting in at very low speed on breaking,what fixed it for me was removing the sensors removing any dirt or grease stuck to the lower face of the sensor, have a look down the sensor hole and see if any serious dirt is lodged in the gaps between the raised teeth and have a look at all the teeth by rotating the drive shaft,finally on reinstalling the sensors,ensure there is a good contact between the sensor body and the inner surface of the hole and or the face around where the sensor retaining bolt is installed