You may still have air in the ABS pump itself. I've heard that activating the ABS a few times helps to clear air from the system (always assuming your back brake is strong enough to trigger the ABS, of course!) The ABS unit can be bled, either by loosening the banjos or fitting a bleed nipple.
As a last resort, it may be corrosion in the master cylinder. See here for more details:
http://www.ukmonster.co.uk/monster/s...ad.php?t=55504