Some 2004-07 Prius hybrids may produce an annoying squeaking sound when the brake pedal is applied or released. Toyota says the noise is most audible when the key is in the Ready mode with the vehicle not moving. It seems that a defective brake actuator assembly is the likely culprit.
Before condemning the actuator, however, try bleeding the brake system. If the noise goes away, you’re done with the fix. If the noise remains after getting out that last belch of air, replace the brake actuator with an upgraded unit, Part No. 44500-47090.