Brake Grinding Noise problems of the 2007 Lincoln Navigator

Purchased vehicle as a cpo on 9/27/08. Vehicle was left at dealership to replace navigation system failure. Took physical delivery and drove home on 10/4/2008. Low tire pressure warning light came on after 15 miles leaving dealership on way home(none of the tires were low on pressure). Returned vehicle to dealer to have system checked and light reset. On 10/29 mileage on vehicle was 19,758. Vehicle was being driven by my wife on a flat city street in laguna hills CA approximately 10 am approx 40 mph and the day was clear and warm. She applied the brake pedal to slow down and the brake pedal was hard and would not depress and the vehicle would not slow down and a grinding noise came from the front wheels and the front end was shaking the anti-skid light came on (while she continued to press down on the brake pedal) and the brakes did not engage until the vehicle traveled approx 100 feet. The vehicle slowed and she pulled into a parking lot at a slow speed and it happened again. Towed to closest dealer. That dealer found no problem, and no codes showed up. Took it to dealer that sold the vehicle to us. They can't replicate the problem, but are still researching.