Every time you push the brake pedal, fluid from the master cylinder transfers to every wheel, thereby creating pressure to stop the vehicle. If the fluid level is low in the reservoir, every brake pedal will push air into the wheels rather than brake fluid. Therefore, you have to push the pedal to the ground to make it work. It will diminish the performance of the braking system, plus it is dangerous for your life. It is not only important to fill the fluid reservoir but keep some with yourself in your vehicle.