Do you mean the red warning light on the dash or the brake lights on the rear of the truck?
If the rear brake lights are on, suspect a brake light switch out of adjustment.
The red warning light will turn on if there is a difference in pressure in the front and rear hydraulic systems, (that switch can trip during improper bleeding procedures, and it can stick even though it's spring-loaded). It will also turn on for low fluid level in the reservoir, and when the parking brake is applied. Since the level is acceptable, suspect a stretched or sticking parking brake cable allowing the pedal to not release completely. Pull up on the parking brake pedal with your foot. If the light still does not go out, the switch could be bent. A quick trick to tell if this is the case is to watch the light very closely while the ignition switch is on but the engine is not running. Slowly push the parking brake pedal down an inch or two. If you see a barely perceptible increase in brightness of the light, the parking brake switch is working.
Next, unplug the fluid level sensor and the combination valve to see which one makes the light go off. I don't recall seeing a level sensor on early '90s Dakotas so I'm leaning towards the parking brake switch.