79cadillac brake light

your question is a bit vague but I shall do my best . If you have no back brakes at all : check the master cylinder , make sure there is fluid on both sides of the divider . if fluid is present only on one side ,your master cylinder rear seal is bad or you have a line leaking or a wheel cylinder blown out . if fluid is present on both sides , pull off the rear wheels and check the shoes for wear . if shoes check out ok , the self adjusters are probably stuck . if so it's best to take it to a shop . if the brake lights are not working check the switch (the white one , the grey one is for the cruise control ) on the brake pedal arm . If all the original equipment is still in place it probably needs a complete brake system overhaul . hope this helps. PBN

ckeck the fuses