Is your pedal hard going down? if it is then is is probabally the accumulator . mine would do that and the brake light would go on then off. after i fixed it my brake light would come on when the pedal would go down all i had to do was bleed the brakes. Also if your powermaster can't keep up with the ammount of time you push the brakes then that would make the brakelight come on. I don't know if any of this would help but they are very common problems that ive noticed with my car
Hope this helps
Andy