2019 BMW M850i xDrive Brake Master Cylinder Reservoir Replacement Cost

Since the brake system on most passenger vehicles uses hydraulic fluid to actuate the service brakes, there must be a steady supply of brake fluid on the vehicle to compensate for brake component wear. This is the job of the brake master cylinder reservoir, otherwise called the brake fluid reservoir. It sits on top of the brake master cylinder, and holds enough brake fluid to prevent the master cylinder from sucking in air when the brake pedal is pressed during any driving condition.

Like other reservoirs, the brake fluid reservoir is mounted on top of the master cylinder, since this is the components that the brake fluid reservoir feeds. There is a tight seal that is both dust-proof and leak-proof which connects the base of the reservoir to the top of the master cylinder. The fluid in the reservoir is not under pressure, but the reservoir is sealed against foreign contaminants. When the brake pedal is pressed, the pistons inside the brake master cylinder are pushed forward causing the fluid in the master cylinder to be pushed into the brake lines under great pressure. Since the master cylinder has the brake fluid reservoir feeding in new brake fluid at all times, the master cylinder will be backfilled when the pedal is pressed. When the brake pedal is released, the excess fluid created by the backwards movement of the master cylinder pistons will move back into the brake fluid reservoir. Some of the fluid from the brake fluid reservoir will slowly be moved into the brake master cylinder and brake lines. This is because as brake pads, rotors, shoes, and drums experience physical wear, they become thinner, and the hydraulic system becomes larger in volume. So, the brake fluid reservoir feeds brake fluid into the master cylinder by gravity as the brake fluid is needed. Note, this is always a gradual process over many miles.

The most common type of failure for a brake fluid reservoir is leaking from the cap. Especially for brake master cylinder reservoirs that do not use a twist-on cap, the seal between the brake fluid reservoir and cover will degrade and fail. This causes leaking down the sides of the reservoir, and dark colored brake fluid inside the reservoir. Additionally, the reservoir may crack or leak at the base where it is connected to the brake master cylinder. In both cases, leaking would be evident, but when the leak stems from a bad seal between these two components the brakes will begin to feel spongy or squishy, and pressing the brake pedal rapidly may be required to build any kind of brake pressure. This is due to air entering the brake master cylinder, which causes severe braking inefficiency.

Any problem with the brake system from squeaky brakes to total failure should be inspected immediately. When attempting to stop a vehicle with a faulty brake master cylinder reservoir, the likelihood of brake failure is high, and the parking brake is not a sufficient substitute for the service brake in this situation. This can pose a serious hazard to passengers and other vehicles on the road. If the brake warning light illuminates, the vehicle should not be driven, as a brake fluid leak will eventually cause total brake system failure.

The failure rate of the brake master cylinder reservoir is more linked to the material used and time than mileage. Since the brake master cylinder is a non-moving part, it really has no way to fail other than deterioration, corrosion, and physical damage. Brake master cylinder reservoirs that are part of the master cylinder, and cannot be separated are likely to last between 125,000 and 150,000 miles, but this is due to brake master cylinder failure. Brake fluid reservoirs that can be separated from the brake master cylinder are likely to fail once they have aged significantly, especially due to seal degradation. Expecting any seal to last more than 10-15 years, depending on the operating environment, may be unrealistic.

Simply put, the brake master cylinder reservoir must be visually inspected to determine failure. If the technician notices the brakes feel spongy or the brake pedal sinks to the floor, he may attempt to bleed the brakes before determining where the leak in the brake system exists. In many cases, even when the reservoir can be replaced separately, the brake master cylinder and master cylinder reservoir will be replaced at the same time. This is most common when the brake fluid reservoir to master cylinder seal is deteriorated. If no brake issues are noted, the technician will note the location of any leaks or cracks, and recommend replacement of the reservoir, seals, or master cylinder and reservoir assembly.

Depending on the components being replaced, this can be as simple as removing the lid and placing a new lid on the reservoir, or it can be as difficult as replacing the master cylinder as a whole. The worst case scenario will require the brake master cylinder to be removed, so let's take a look at how that is accomplished. First, the technician will begin by draining the brake fluid from the system, especially since the new brake master cylinder will require new fluid and bleeding of air. Then, while the brake master cylinder is still firmly attached to the brake booster, the brake lines will be removed, and excess fluid will be caught with rags or a drip pan, ensuring that the brake fluid is not allowed to drip on any painted surface. Next, fasteners which attach the brake master cylinder will be removed, and the brake fluid level sensor and brake pressure switch will be disconnected if equipped. Afterwards, the new brake master cylinder with the attached brake master cylinder reservoir can be mounted, electronic switch connectors installed, and brake lines can be installed as well. Lastly, the reservoir will be filled with the appropriate grade of brake fluid, and the entire system will be flushed of the old brake fluid and bled of air before testing.

We recommend using the factory approved brake fluid for the vehicle being repaired, and not mixing any types of brake fluid. Also, the brake system should be flushed and inspected for obvious signs of deterioration, and repaired if necessary to prevent further failure. If the Entire brake hydraulic system is replaced, a new type of brake fluid may be used, but it must be at least the minimum standard of when the vehicle was manufactured.

Mixing brake fluid can lead to brake system failure due to congealing of the fluid within the brake lines. The different types of brake fluid, i.e. DOT3, DOT4, or DOT5 are meant for higher and higher braking demand, and will not always result in better braking performance. In fact, except for the most strenuous braking conditions, upgrading brake fluid will likely have no benefit.

Brake master cylinder reservoir replacement is an easy task given a good knowledge of hydraulic system maintenance and operation. The informed and experienced DIYer can tackle the diagnosis and repair of the brake master cylinder reservoir in a few hours, but proper bleeding and diagnostic procedures must be understood and followed. If the brake master cylinder must be replaced along with the brake fluid reservoir, or if diagnosing the cause of brake system inefficiency proves too difficult, have a professional tackle this job as a safety precaution.