Why does the ABS light come on once I reach a speed of 15 mph? on 1993 GMC Vandura 2500

I also have the instance of the parking brake light coming on periodically. If I turn the ignition off and restart the car it goes out. What's causing that to happen and how do I fix it?

Asked by for the 1993 GMC Vandura 2500
The ABS computer checks wheel speed sensor signals once the vehicle is rolling and may detect one of the sensors has a bad pattern due to a bad sensor or sensor tone ring. A scan tool is needed to retrieve the codes stored. Check the brake fluid level ensure it is correct.There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate
Patrick, thanks for the info. Hopefully I can get this mystery resolved.