[spike88]

As first fix, I would bleed the brakes. Sounds like air in brakes or air bubble within ABS unit. If that doesn't work, get a reader and "read its code". Its code will advise if sensor problem or some other problem. Bleeding brakes with 2 people should be easy. re: One person to press the brake peddle and a different person to wrench its bleed screw (on further wheel hub working closer to Master Brake Cylinder).

Good luck...