Anti-Lock brakes

I have a 99 Blazer 4x4. When braking at a slow/idling speed the pedal will sometimes gets hard. After tapping the pedal the brakes return to normal. Feels like just the front left front caliper goes into anti-lock mode. Any suggestions? I was thinking maybe replacing the Caliper? The pads and rotors were replaced and the problem is still there.

It is something besides a caliper problem.
Speed sensors will do things like that.
Is it turning on the abs light?
Can you get it checked on a ABS capable scanner?
MT