brakes loose power

Hello, can somebody help me? I have a 1994 passat. the problem Iam having is after I start the car and drive for 1/2 hour the abs and brake light goes on and I loose braking power and I have to press hard on the brake to stop. The manual says check for low fluid but its full. has anyone had this problem? The dealer says they have never seen this problem, I dont believe them!