2004 BMW 645Ci Brake Master Cylinder Replacement Cost

When the brake pedal is pressed, the master cylinder pushes fluid under high pressure to the brake calipers, forcing them to squeeze the brake pads into the brake rotor. This pressure is created by two small plungers inside the master cylinder that compress the brake fluid as the brake pedal pushes them forward.

The term 'spongy' is well known in the automotive field, and describes the feeling of the brake pedal when air has been introduced into the master cylinder, or the master cylinder has pushed air into the brake lines. The brake pedal may also sink to the floor when pressed and held. This indicates the plungers inside the master cylinder are leaking internally, and will not produce the pressure needed to cause the vehicle to slow or stop. Learn more about brake master cylinder symptoms

It is not safe to operate any vehicle with any braking problem. If there exists any sign that the brakes are not performing as originally intended, the vehicle should immediately be placed out of service, and repaired. Collisions result from brake system failure, and the parking brake should never be substituted for the normal service brakes.

Replacement of the brake master cylinder is a common repair to many makes and models, however, the majority of vehicle's will not need this repair before 125,000 miles. This is because there are very few moving parts, and the system is sealed from air, dust, and other brake fluid contaminants. Each manufacturer protects their customers from master cylinder failure by providing a schedule for maintenance for the entire braking system. This maintenance includes flushing the brake fluid, cleaning, and inspecting components, as well as replacement of wear and tear parts. Following these recommendations may even ensure the master cylinder will last the lifetime of the vehicle. However, neglecting maintenance specified by the factory may cause premature wear of the master cylinder plungers, leading to an early onset of the mentioned symptoms.

The operation of the brake pedal is the best indicator of master cylinder failure. If the pedal falls to the floor when pressed, little or no braking force is being created, and there are no external leaks, there is a strong possibility the master cylinder has failed. If the brake pedal becomes firm after pressing quickly and repeatedly, but returns to the floor when held, the master cylinder will most likely be at fault.

After the brake fluid is drained from the system, the brake lines must be removed from the master cylinder. There is normally one electrical connector to remove, and finally, the master cylinder can be unbolted and removed from the brake booster. Replacing the master cylinder is the reverse order, but the master cylinder must be bled of air prior to bleeding the air from the rest of the braking system.

If the master cylinder has failed, there is a possibility of rubber particles floating through the brake lines. Rubber can be compressed, whereas brake fluid cannot. This means that rubber particles in the brake lines can produce the same symptoms as air, and a spongy feeling brake pedal may result. To avoid this, the brake system must be thoroughly flushed to return the brakes to normal operations.

Master cylinders, as noted above, have two plungers that push fluid. If one of them fails, either the front or rear brakes may work properly, while the others will not. This will greatly reduce stopping power. If braking ability has diminished, the vehicle should be towed to a repair facility, and only driven once repaired. Mixing brake fluid types can cause complete brake system failure. The type of fluid noted on the master cylinder is the only type that should be introduced to the system. Normally DOT-3 brake fluid is used, but this varies by vehicle. Brake fluid is corrosive to automotive paint, and can cause blindness if contact with the eyes occurs.

The braking system can be maintained by a DIYer with a complete knowledge of hydraulically actuated systems. This is important as testing brake operation, and safe installation of braking components rely on this knowledge. If the complete braking system is not understood, damage to the expensive braking components, or injury is likely. Trusting a professional technician with the replacement of a master cylinder is normally the best course of action.