Brake Antilock Wheel Speed Sensor problems of the 2006 Toyota Prius

When braking quickly usually on rough surfaces, wet roads, slopes brakes temporarily fail as if no control over the car giving me an adrenaline rush. I have been driving like this for 4 years thinking is the characteristic of Prius. June 30th 2011, I steered into the parking lot and was parking on the first spot at the right immediately after entry to parking lot at the speed of 5 mph. Approximately half way to curb I withdrew my foot off the gas pedal and put on the brakes slowly. I was expecting the car to halt prior to the curb line as usual. But it did not stop on the contrary it accelerated and went over the curb and right on the fence. The fence was approximately 2 feet from curb; before it went on fence I looked at my feet and made sure it was on the brakes as could not believe what was happening to the car and yes I am 100% sure it was on brakes. I also applied parking brakes and put car in neutral and shut off the power, when it stopped over the fence. I have reported this to dealer as the car had been serviced 4 weeks prior to this incident at 32,431 miles. The dealer did health check scan twice taking 6 hours for each scan. After first scan I was told the car had front speed sensors codes and a low or abnormal high battery voltage. And advised to detect sensors in details and replace them at next appointment. Second day after the manager did another health check for 6 hours, I don’t know why another scan; and reported codes for low battery and speed sensors and ecu. None have potential to cause customer concern. They finally blamed the floor mat which was definitely not the problem. Surprisingly when I asked how did they take care of the codes? if sensors need to be replaced, they told me the codes are gone now. There were no details provided inspite of me asking several times why the car moves after applying brakes. Being a doctor I am concerned with safety and prevention more than anything.