Brake Light On problems of the 2008 Chevrolet Express

This is a 2008 Chevrolet 3500 cutaway school bus with 24,926 miles on. The driver was coming to a stop traveling approxiamately 15 mph. He applied the brakes and the pedal went to the floor. The bus then rear ended a another van which then hit the rear of a postal truck. The drivers of the two other vehicles were taken away by ambulance complaining of back injuries. The bus was examined at the scence of the accident by two penndot class 7 inspection mechanics verifying that the brake pedal went to the floor. The tow truck driver (third party tower) also checked the brakes and opened the master cylinder to see if the fluid was down or empty and stated that it was full. The bus was towed to our garage where we moved the bus and then had a brake pedal. There were two lights on the dash, the abs light and the "brake light" (pictures taken). We read the codes (obd ii) and had c0035 - left front wheel sensor circuit malfunction, and c0040 - left front wheel speed sensor circuit malfunction (pictures taken). The bus was driven around and did have brakes. This same bus was at the local Chevrolet dealer November 09,2010 to have both front abs sensors replaced (dealer work order available). It was taken to the dealer because of continued complaints of the brakes failing to stop the bus. As the drivers described the problem - " it was like trying to stop on ice". The sensors were replaced and the bus was put back in service. This is a very unsafe condition, particuarlly since this is a school bus!.