If your fluid is not low, do you have cruise control and if so does it fail when this light is on? Check your master cylinder for a pressure switch. It might be coming right off the master or it might be inline somewhere. These things have/had a habit of doing a slow failure where they would leak internally and randomly short out causing ABS lights, brake lights, CC failures and what-not until complete failure. I've had the same failure on 3 different Fords from a '92 up to 2001, a Superduty P/U, an Excursion, and an E350. There are various Ford TSBs out there addressing the issue. Cheap easy fix each time.