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.