turns out it was the mapp sensor, now im just working on getting the brake light mil on the dash to turn off. i am not able to figure out what it could be. i wish the reader i have worked on this vehicle (the one i have is obII) but even then its not set up for reading brake system codes

 

brake light(red)? is simple. check your master cylinder for leaks and check your brake lines for leaks. if they are solid then it is your combination valve or proportional valves which are mounted directly under the master cylinder bolted to the frame. Red brake lights only come on if there is a fluid loss or loss of pressure or signal.

or it could be a stuck E-Brake pedal.

brake light(yellow) is an ABS issue. your truck is so old who cares if you dont have ABS. but even in this instance all it could be is independent wheel sensors or ABS control unit, nothing more in 1990, unless you had a Cadillac.