The pain just keeps on coming for Toyota. First came complaints of cars running away without the driver pressing the accelerator. Now the Japanese government has ordered Toyota to investigate the braking system on the 2010 Prius. The U.S. government is also looking into the matter after receiving over 100 complaints about the brakes on the third-generation hybrid. Like most other hybrids, the Prius uses an electro-hydraulic brake system that blends the regenerative and friction braking functions. There have apparently been dozens of complaints in Japan and North America of a momentary loss of braking power at low speeds on slick or bumpy road surfaces.
We can't say for sure what is or is not happening on the Prius, but we can offer some possible insight. On hybrids vehicles, the brake system monitors the state of charge of the battery, the available amount of regenerative braking and the driver's brake demand (as measured by the brake pedal apply). As the vehicle comes to a stop, the amount of regenerative braking is ramped down to zero since you can only capture kinetic energy when the wheels are rolling. Doing this smoothly requires an accurate software model of the friction braking behavior. If the friction brakes are generating less torque than the model says it should be, the brake system will apply less pressure to the brakes. The result would be a momentary loss of deceleration during the hand-off from regen to friction braking.
Because the behavior of friction brakes varies over time depending on temperature, moisture, wear and other factors, it is very difficult to calibrate these models. The other part of this equation is the fact that this is happening at low speeds. The wheel speed sensors become less accurate at low speeds as the time period between sensor pulses is extended. Rough or slippery surfaces exacerbate the problem, which could make the brake system think there is slip that might not be there, resulting in lower brake pressure. The result is a perceived loss of deceleration.
If these are the problems that the Prius is experiencing, they could be corrected with an update to the control software. However, that will take time for Toyota to calibrate and validate.