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Mounted at the firewall with a network of thin steel brake lines running to it is the brake master cylinder. It’s a crucial component for safe driving since all of your brake operation depends on the master cylinder working as it’s designed. But like any car part, there’s potential for the master cylinder to fail, and understanding what to look for can reduce the likelihood of a problem coming to a stop. 

The cost to replace a master cylinder ranges from $250 to $600 in most cases, although some parts that integrate electronic modules or sensors can be much higher. It’s not exactly an expense you want to incur, but it’s important to fix when necessary.

What is a master cylinder’s role, and what can you expect when it’s failing? Here’s what you need to know.

What is a Brake Master Cylinder? 

A master cylinder is a hydraulic device that helps push fluid through the brake lines to the brakes themselves. The master cylinder contains two chambers, each with a piston. These pistons are connected to the brake pedal. When you press the brake pedal, the pistons push fluid from the chambers through the brake lines to the brake calipers or wheel cylinders, making them press the brake friction material against your rotors or drums. 

Some parts are very closely related to the master cylinder too including:  

  • The fluid reservoir. There must always be brake fluid in the reservoir, otherwise air can enter the brake lines that needs to be bled out. 

  • The brake vacuum booster. This vacuum-assisted part helps minimize the brake force required to push the pistons in the master cylinder. 

  • The brake proportioning valve. This hydraulic valve helps direct the appropriate amount of pressure to the front and rear wheels respectively. 

How to Tell if Your Master Cylinder is Going Bad 

All of your braking operations hinge on the brake master cylinder working well. Your driving safety depends on it. If your car’s master cylinder is going bad, there are several warning signs to watch out for.  

  • Brake fluid is low: A car’s master cylinder is responsible for supplying hydraulic pressure to the brakes. If it is leaking fluid, that pressure is reduced, and the brakes may not work as well as they should. You’ll notice that the fluid level in the reservoir is low or empty, and that topping up the fluid isn’t a long-term solution.

  • Brake pedal sinks or feels spongy: A spongy or soft brake pedal can be caused by a leak in the system, and it can cause your brakes to fail. When the brake pedal sinks, that usually means that the fluid is being pushed out of the system with pressure, and when the pedal feels spongy, it indicates the presence of air in the lines. Either way, it can change how your car’s brakes perform.

  • Contaminants in the brake fluid: Another warning sign is if your brake fluid looks dirty or contaminated. It should look a light golden color or clear, but it does get darker when it has contaminants in it. These can be from the master cylinder allowing moisture into the system or, more likely, there are metal shavings from the master cylinder failing internally.

  • Grinding noise when brakes are applied: If your car is making a grinding noise when you apply the brakes, it could also indicate that the master cylinder is going bad. The noise itself is from worn-out brake material, though, not the master cylinder itself. It’s possible that there’s more pressure to the front or rear brakes than normal, wearing them out prematurely and causing the grinding noise.

  • Car pulls to one side when brakes are applied: When the car pulls to one side when brakes are applied, it could also be the master cylinder. In a situation like this, the car’s brakes aren’t getting pressure evenly on both sides, and the brakes grab and pull the car one way or the other.

How to Deal with a Bad Master Cylinder  

The best way to deal with a bad master cylinder is to replace it as soon as possible. A bad master cylinder can cause your brakes to fail, which can be extremely dangerous. If you’re experiencing any brake problems, make sure to take your car to a mechanic right away to have it checked out or change the master cylinder yourself.  

Master cylinder replacement will take a couple of hours if you prefer to do it yourself, but it’s not exactly a difficult procedure. With the parking brake set and the transmission in park, suck any remaining fluid out of the reservoir and crack the brake lines loose with a wrench. Then, remove all the mounting bolts, the brake lines, any sensor connectors, and any other parts connected to the master cylinder. You’ll need to unclip the piston rod from the brake booster next.  

Install the new brake master cylinder, then fill it. Always use brake fluid matching the exact spec on the reservoir cap or your owner’s manual, and only from a new container. Once it’s installed, you’ll need to bleed the air from the brakes at each wheel, making sure the reservoir never empties and allows more air in.  

Buy your replacement master cylinder at AutoZone along with any related parts and fluids you might need. Unsure of the parts or tools for the job? Ask an associate for Trustworthy Advice. 

If you decide that it’s too big a job to tackle on your own, let AutoZone help you find qualified professional mechanics through our Shop Referral Program

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