1981 Cadillac Deville Brake Problem

I own a 1981 cadillac deVille with a V6. The problem is that the brakes are hard and will not stop like they should. Once in a while they work ok. But when they get hard and hardly move towards the floorboard I really have to push very hard on the pedal in order to come to a complete stop. Kind of scared me the first time it did it. Some one told me to replace the brake booster and the master cylinder. What is your opinion? Any feedback would be appreciated. Thanks.?