It was a post by Steve. It has to do w/ the front and rear tire sizes being within a certain ratio % from each other. Stray too far and you get and ABS light, I thought. Doesn't sound like the OP is dealing w/ an ABS light though. From what I gather it appears to be working fine (hence the front is working), just sounds like the rear tire is locking up if he gets on the rear binders too hard. A harder tire compound like a CT skid a little easier, but if the ABS is working correctly, then when the rim stops rotating , it should pulse the rear brakes to prevent lockup.