Dont get me wrong, what are we talking about? A bullnose ( 80-86) does not have ABS. Is the BRAKE light on, on the dash? If it is, it could be a stuck switch on the parking brake. It could be the proporting valve stuck in one side or the other because of a brake problem, meaning something is out of balance in the brake system.
Steve
86 Bronco, XLT, 5.0 EFI, EEC IV, AOD, IFS, limited slip front and rear, 3.08 gears, Tilt steering, factory AC