As its name suggests, the brake master cylinder is the single most important component in the braking system of a car, controlling the brake functions through hydraulics. A bad master cylinder can even cause the brakes to fail on your car, if left undiagnosed. To learn more about brakes and comparison of braking performance in modern cars, CarHP has a dedicated sub-section in each car review so that you can make a better purchase decision.

Guide to Check Your Brake System’s Master Cylinder

Master Cylinder

Braking System Maintenance

According to the NHTSA, more than 20% of road accidents are caused by brake-related issues. So it is recommended to periodically check the whole braking system for any leaks or damages periodically. Even if the system is working inefficiently, braking performance can be affected.

Fortunately, checking and maintaining the braking system is straightforward. There are some symptoms to watch out for, indicating a compromised braking system. Spongy brakes are one of the most common symptoms indicating an issue with the master cylinder; brakes would feel spongy on application, requiring more force than usual. The master cylinder is used for sending brake fluid to the calipers through the lines, causing the brake pads to squeeze against the discs, creating friction and slowing the car down. However, this could also indicate a leak in the brake lines or other components.

Checking The Brake Fluid

One of the simplest and most important checks you can do is the brake fluid reservoir. Open the hood and locate the reservoir connected to the master cylinder. It is usually a small plastic bottle on top with markings on the sides. Lift the cap and check if the fluid is between the max and min mark. Depending on the brake pad wear and other factors, the fluid can get lower in some cases. Care must be taken to avoid getting dirt or other debris into the fluid when checking it. Also, check the color of the fluid. Use a flashlight to check all the surrounding areas and inside the master cylinder for any gunk buildup or other debris. The cap will also have a diaphragm that pushes down the fluid when closed. Make sure they’re in good condition.

If everything checks out, replace the cap carefully, without letting any dirt into the fluid. Wipe the cap down as an extra precaution before installing it. Most modern cars have brake fluid sensors that will detect low fluid levels and alert the driver through an indicator on the instrument cluster. Some models use the check engine light to warn the driver. In this case, checking the fault codes will reveal the issue. It is recommended to replace your brake fluid every 2 years for optimum braking performance and to avoid damaging the braking components including the master cylinder.

Applying Brake Pressure

Applying Brake Pressure

Open the brake fluid reservoir and have someone depress the brake pedal. If you notice bubble formations in the reservoir, it indicates a malfunctioning master cylinder. When applying maximum brake pressure, if the pedal drops down slowly, it could also indicate an issue. A metallic scraping sound can also be heard when depressing the brakes in some cases. Moreover, if the brake pedal sinks, there is usually a fault with the seals inside the master cylinder, causing the fluid to seep out.

Contaminated Brake Fluid

As the car ages and if the brakes are not bled frequently enough, the brake fluid can get a lot of contaminants in it. Worn or damaged seals can contaminate the fluid along with rust, dust, and more. Contaminants in the system can affect the braking performance, making the pedal harder. It will also make the brake fluid darker. New and good condition fluid should be clear with a slight yellow tint. If the color changes to dark brown or even black in some cases, the fluid is contaminated and requires replacement. Contamination will also lead to gunk formation at the bottom of the reservoir or the cap.

Replacing The Master Cylinder

If the system has a leak or doesn’t perform as it should, the safest option is to replace the master cylinder assembly. However, depending on the make and model of your car, it can get expensive, going up to $1000 in some cases. Replacing the cylinder will also require a thorough brake bleed which can cost an additional $100 or so. In some cases, the master cylinder can be repaired and used again by replacing the seals, saving you a lot of money. Replacing the seals typically costs around $300.

Bench-Testing The Master Cylinder


One of the best ways to thoroughly test a brake master cylinder, especially new ones, is through a bench test. A rebuilt master cylinder with new seals can also be tested for its effectiveness. The assembly is mounted on a vice and pressure is applied to the plunger. If the plunger stays in place, it means the cylinder is good and is capable of holding pressure. However, if the plunger keeps moving or is not strong enough, it indicates an internal issue with the master cylinder assembly. Before installing a new or rebuilt master cylinder, they should be bled to remove any stored air inside the cylinder.

Because of how important the brakes are in a car, it is recommended to keep close tabs on the braking system and check the fluid levels once in a while. If any abnormalities are felt during braking, it should be checked as soon as possible. The brake fluid should also be replaced every two years, depending on usage. Care should also be taken when handling brake fluid as it is highly corrosive and can damage the paint and other components. Along with the brake fluid, also check the brake pads and rotors to see if they need replacement.

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