1998 Jeep Wrangler brakes problems

I was approaching a red light and approximately 3 car lengths away from the vehicle already stopped at the light I began braking and heard a noise under the floor board and while pushing the brake pedal all the way down and the braking system did not slow or stop the vehicle at all. I lifted the brake pedal from the floor and reapplied it and the vehicle did not slow down or stop until it ran into the back of the other vehicle.

Dt: no rear brakes and the front brakes locked up. The trouble takes place mostly when it snows or when it is slippery. The back brakes have never worked. Dealer changed everything in the brake system, except the master cylinder. Inspector from jeep came ,. And worked on it. Then, they said the nature of the beast was not to have any rear brakes.