Your using a DFGT aren't you? I had a Driving Force Pro (I believe the pedals are the same between models) and I had trouble with accurate braking with ABS off with the DFP. Since having the G25 with the much improved pedals, racing ABS=0 has become a lot easier, being able to accurately adjust the brake pressure as need be.
Although, competatively in GT5Prologue, most people run with ABS=1 due to the way the ABS is modeled, not only is it easier, its faster.
EDIT: For setting up brake bias for ABS=0 start with Brake Bias Front=3, Rear=1. That should help by reducing lock ups, you also have to learn as you brake in a straight line, use as much of the brake pedal as it will allow, and as you begin to turn in, ease off the brakes as you wind up the steering, the more you turn the wheel, the less you have to be on your brakes or you'll lock 'em.