Brake Booster

WARNING: This page is about a different car, the 2011 Nissan Leaf. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Identifying Motor And Piston Of Brake Booster

Courtesy of NISSAN MOTOR CO., U.S.A.

Contains a motor and generates boost force according to the amount that the brake pedal is depressed and the amount of cooperative regenerative brake control.
Uses the boost force to generate fluid pressure in the master cylinder.