Two problems related to brake sensor have been reported for the 2011 Toyota Prius. The most recently reported issues are listed below.
Every once in a while - it can be months between events - our Prius momentarily slams on the brakes when driving at highway speeds while nothing else is going on. We know it's not operator error. It's happened to my wife several times and me once. When it happened to me I was driving straight ahead on a dry, lightly traveled divided highway during the day. Everything on the dashboard lights up, alarms go off, the car decelerates like I suddenly stomped on the brake pedal though my foot never moved off of the gas, it lasts for maybe a second and then the car is back to normal. It's the same each time it's happened. Luckily each time this has happened we haven't had anyone tailgating us. We've taken it to the dealer multiple times. The first couple of times they looked at us like we were loony. They checked it out and we got it back with a message that said the technician checked the computer and test drove it for 20 miles and the car was operating as designed. Except it happened again. We finally convinced them that this was maybe a legitimate issue and they said they brought in a Toyota field specialist who checked the car out and they said that the issue was the pcs wave radar sensor that was slightly out of alignment due to body damage (my wife had hit a small deer about a year prior to that - damage was minor and repaired at a local body shop). After they realigned the sensor, the car behaved itself for about five months. Until yesterday when it happened again.
The abs braking system in this car is overly sensitive. This does not happen often, but it has happened enough over the past three years that I can clearly predict when it will happen again. If you are slowing down moderately say for a traffic light, on a dry road, but one that is in need of repaving (no pot holes, but just a rough surface), and if you are on a road that is going slightly downhill, the abs braking system kicks in without a need. This causes your foot to sink briefly to the floor (on the brake pedal) and give the feeling of a loss of braking for a few seconds. The brake pads are fine (even with such high mileage, I still have over 70% of the original brake pads left). I have brought this to the attention of my Toyota dealer, who says everything is fine. The scenario described above might only happen once every few weeks, but both my wife and I have had this happen numerous times. I believe it is a software issue, perhaps related to overly sensitive sensors, perhaps in the rear of the car.