91 AntiLock Brakes

Well They don't work. I was driving down the street when it was raining and came up to a stop sign. They locked up and i just slid. go thing i was only doing about 5-10 mph when they locked up. If there were any people in the intersection i would have been fu*ked. But the dummy light is on. What could be wrong with them, and what should i look for?