ABS Brake Light

Hi,
I have a 91 Integra GS. Lately the ABS brake light is coming on. The perplexing part is that it only comes on after driving for a few miles (5-7), and stays on. Any pointers on why and what to look for?
V

There is an ABS sensor in each wheel, so for most of early 90s models, the sensors do not work well, and maybe one of them is damaged. To check what's exactly going on of each sensor, a special tool is needed and only dealer has it.