One problem related to brake sensor has been reported for the 2003 GMC Sierra. The most recently reported issues are listed below.
I was approaching an intersection where traffic was stopped for a red light. I applied the brakes and was decreasing speed from approx 35 mph. At approx. 10-15 mph the brake pedal went to the floor without any warning or sensor detection and the brakes failed. I held the pedal to the floor, immediately shifted to low gear and drifted to the right shoulder to avoid the stopped traffic. I slowly moved the vehicle to a safe place. In a short distance the service brake system warning activated but there was no warning prior to failure. The truck was taken to my mechanic where it was found that the brake line on the left frame rail had failed under pressure due to extreme rust. It was also determined that all the brake lines, emergency brake cable and brake plates were rusted far in advance of expectations for my model year and mileage. All brake lines have to be replaced because another failure is certain if only the blown line is replaced. I contacted the dealership that sold and has serviced my vehicle and they stated that there was no assistance or recall for rusted brake lines even though there are numerous complaints.