It is the Chevrolet brake master cylinder where the pressure applied by the brake pedal compresses the hydraulic brake fluid. The brake master cylinder is mounted to the vacuum brake booster on the driver's side of the firewall under the hood of your Chevrolet. On the top of the brake master cylinder is a reservoir either covered by a snap-tight lid or pressurized cap. The brake fluid reservoir should be checked periodically so the proper level can be maintained. Over time the fluid will decease on your Chevrolet. This is due to the fact that more fluid is needed in the system as the brake shoes and pads wear. If this brake fluid is not replenished, air could become trapped in this closed loop system allowing for moisture to enter it. You can also experience a reduction in the braking potential of the system if air enters. The failure of the Chevrolet brake master cylinder will occur if the seals on the piston of this device fail. Unlike other seals on your car that fail, there may be no leak on the exterior of this part. The driver of the car will experience the brake pedal moving towards the floor of the car without an increase in the braking power of their vehicle. This is a symptom of the seals failing in the brake master cylinder. The only way to fix this problem is for the replacement of the brake master cylinder. It is true this part can be rebuilt; most shops today do not have the proper equipment to conduct that type of rebuild. If you experience the symptom described above on your Chevrolet, take your car to the shop before total brake failure occurs. Once the seals in this part reach total failure, you will not be able to stop your car.