The brake servo motor is a valve that is actuated when the brake pedal is depressed reducing pressure on the master cylinder side of the servo. The higher pressure on the other side assists the brake actuator push rod to apply force to the piston in the master cylinder.

When this device gets damaged, your brakes will be very difficult to engage and after a few miles, your legs would be worn out. Sometimes a slight hissing noise can be heard when it is getting worn down, but if it does completely fail; your brakes will still work. Brake booster failure will leave a vehicle with a hard brake pedal making it much more difficult to stop.