Check the fluid level and also your brake lines and master cylinder for leaks. Sounds like you are getting air in the system. A buddy of mine just had a similar problem on his 97 and we found the rear drivers brake line had rotted through near the T-fitting on the rear axle. Check to see if the dash bulb is burned out as well. It should light when you start the truck and then cycle off after starting. I may be mistaken, but if you do have a leak you would feel it immediately at the pedal but it wouldn't chip the light until you lost enough fluid to trigger the low level sensor at the reservoir.
1998 Ford F-150 XLT Flareside, 4.2L V-6, 2WD, automatic xmsn.