The master cylinder displaces hydraulic brake fluid under pressure to the rest of the brake system. When the brake pedal is depressed, the push rod moves the primary piston forward in the cylinder. The hydraulic pressure created and the force of the primary piston spring moves the secondary piston forward. When the forward movement of the pistons causes their primary cups to cover the bypass holes, hydraulic pressure builds up and is transmitted to the wheel cylinders. When the pedal retracts, the pistons allow fluid from the reservoir to fill the chamber if needed. Special sensors within the master cylinder are used to monitor the level of the fluid in the reservoir, and to alert the driver if a pressure imbalance develops. The standard dual master cylinder gives the front and rear brakes separate hydraulic systems. If a brake fluid leak occurs in one system, the other system will still operate, making it possible to stop the car.