What Is a Brake Master Cylinder and How to Tell If Yours Needs to Be Replaced

September 9, 2022

If the term "brake master cylinder" sounds important, that's because it is! Your car's master cylinder is a critical brake component. Without it, your brakes simply won't work. That's why it's important to know what a master brake cylinder is, how master cylinders work, and when to visit a Nissan Dealer for brake service.

What Is a Brake Master Cylinder?

The brake master cylinder is the first component in your braking system and is connected directly to the brake pedal. It consists of a metal tube with valves that push brake fluid through the brake lines and down to the wheels. On top of the cylinder is a reservoir that holds brake fluid. Inside the cylinder are a series of pistons, springs, and rubber seals designed to create hydraulic pressure.

How Does a Brake Master Cylinder Work?

When you press down on the brake pedal, a pushrod activates the pistons in the master cylinder, which is filled with brake fluid from the reservoir. As the pistons move and their corresponding springs compress, pressure is created inside the master cylinder. Since brake fluid is virtually incompressible, the pressure from the pistons forces the fluid into the brake lines. This is known as hydraulic pressure.

From there, the brake fluid travels to the brake units at each wheel. In the case of disc brakes, the pressurized fluid enters a brake caliper and causes brake pads to rub against a rotor and slow down the wheel. For drum brakes, the same effect is created when the fluid pushes a set of drum shoes against a wheel drum.

How to Tell If a Brake Master Cylinder Is Bad

Even though master cylinders are designed to last the lifetime of your vehicle, it's not uncommon for them to spring a leak or otherwise break down in some way. When this happens, you'll need to repair the problem right away to ensure your car can brake safely. If you're experiencing any of the following symptoms, it could be a sign that you're due for a master cylinder replacement.

Soft, Squishy, or Sunken Brake Pedal

One of the most common symptoms of a faulty brake master cylinder is a squishy brake pedal. When your brakes are working properly, the brake pedal should depress with some resistance and then hold firm. If, however, the master cylinder is no longer able to pressurize properly ‐ usually due to internal leakage ‐ then the brake pedal will feel abnormally spongy or even sink to the floor.

Low Brake Fluid Levels But No Visible Leak

Your brakes require a certain amount of fluid to function safely. If you notice your brakes are underperforming and the fluid reservoir is lower than usual, chances are you have a leak. Leaks can occur in many places within your braking system, often in the brake circuits or at the calipers. Yet if you check the brake lines, look under your car, and inspect each wheel's brake unit without spotting leaking fluid, you are likely dealing with an internal leak in the master cylinder. Internal leaks happen when the rubber seals around the master cylinder's pistons wear out and can no longer contain the brake fluid.

Contaminated Brake Fluid in the Reservoir

Clean brake fluid is normally clear in color with a yellow tint. If you inspect the fluid reservoir and find that it's dark brown or even black in color, your brake fluid is contaminated and should be changed promptly. Healthy brake fluid can become contaminated when the rubber seals in the master cylinder break down and enter the reservoir, causing the darkened color.

Brake, VDC, or ABS Light Appears

Your car may have a brake fluid level switch or sensor installed in the master cylinder. If these sensors detect a drop in fluid levels, they can alert you with a warning light on the dash. We recommend visiting your nearest Nissan service center to have the vehicle scanned to confirm the problem is with the master cylinder.

Does Your Brake Master Cylinder Need Replacing?

You need a working brake master cylinder to operate your brakes. If you think there is a problem with your master cylinder, you need to have it looked at right away. Bring your vehicle to your nearest Nissan service center, and do not forget to order genuine replacement brake parts at Nissan Parts & Accessories.