I also had similar problems, turned out that the wire to the ABS sensor was being compressed as some idiot had routed it through the suspension spring.
Have a look in the workshop bit on this site as theres a proceedure for pulling the fault codes from the abs ECU which will tell you exactly what sensor(s) are faulty.
A word of advice, I left mine, until when I went to brake and other than a bad vibration through the pedal very little happened braking wise and I had so run the motor up on a grass verge to stop her ! and I was standing on the brakes !