Check that your brake fluid reservoir is full of fluid! My clutch was leaking and while the reservoir is designed to not allow the clutch to draw below the line necessary for the proper operation of the brake, the brake light was still on intermittently. It is a float sensor that checks the fluid, make sure it is not stuck. WillieP is also right in checking the sensor for the parking brake, it is simple wire that when grounded will turn the light on.