Brake Light won't go off

The dash brake light won't go off in my 1989 Bonneville. It used to flicker on, and then once you turned the car off and back on it wouldn't come back on. Now it stays on all the time.
I never use my e-brake, but I checked to make sure it wasn't accidentally on. The light actually came on while we were going down the freeway, not upon starting.

So, is there anything else this could be? I checked in my Hayne's manual, but there wasn't anything suggesting it could be the result of something else.

Check the pressure switch at the master cylinder. The brake light often indicates a loss of pressure in either the front or rear brakes. There is a valve that moves in which ever direction the pressure is lowest in and turns on the light. You could have a leak or just a bad sensor but get it checked right away.