I'll give you my experience with our 2008 Prius whose actuator failed and I replaced. Feel free to take what you can use and disregard the rest. Prius have a brake system accumulator to develop pressure for the brake system. Since a Hybrid's engine runs sometimes and is at rest others there must be a system in place to develop bake pressure when the engine isn't running. The Brake Accumulator has a pump and a accumulation chamber as well as other electronic subsystems. The pump develops pressure that is stored in the accumulator and used when the brakes are pressed. If you car has been off awhile when you approach it with the key fob the whirling sound you hear is the Brake System Accumulator coming to life and pressurizing the brake system in preparation for the car to be driven. The Brake System Accumulator is located on the engine firewall, driver side about 18 inches from the top of the firewall. Our failing Brake Actuators Symptoms were the system pump running like clockwork every 30 to 45 seconds to keep the Accumulator pressurized. Before the system pump would run infrequently and I never noticed it. This change to the noise every 30 to 45 seconds was very noticeable to me. Incidentally the brakes worked fine the system pump was coming on and off because the Accumulator which has a bladder was no longer holding pressure like it used to and the pump had to run much more often to maintain the proper pressure I took it to the Dealer and they confirmed the Actuator was in the beginning stages of failure and that even though it was not currently throwing a code eventually it would. As they predicted eventually began getting three warning lights ABS, Brake System, and a VSC light. as well as code C1256. A new actuator cost $1200 online from Toyota so I decided to order a pre owned actuator and install it. As soon as I received the used actuator and looked at it I immediately knew I was not installing that on my families car for the brake system. I called them up and arranged a return and ordered the new Toyota Brake Actuator. I installed the new actuator and accompanying brake system resistor, bled the brake system, and calibrated the linear offset all with Techstream. This was a big job but it turned out fine. There are some great threads here on Prius Chat and You Tube videos that can help you out. I hope my experience can give you a starting point. The Dealer wanted $3000 for the job This is a very common problem with Generation 2 Prius. An article you might find interesting Brake Actuator Failure | Hometown Hybrids
The pump runs more and more and the barking sound when pressing the brakes hard, also loss of brake pressure, its a good idea to test the brakes if you feel its going and as long as its working fine, you shouldn't need to replace it until the lights hit or failure of the brakes occur. Ive seen them start making these sounds and stay that way to 180K+ and then finally fail, others fail 130k, its really a hit a miss.
