Top 4 Bad Master Cylinder Symptoms

Bad Master Cylinder Symptoms

Picture yourself driving down the road. All of a sudden, you hit the brakes, and they feel off. They're either spongy or soft or not responsive at all. It's definitely a nerve-racking situation, especially if you're unsure what's causing the problem.

Well, one of the reasons could be a bad master cylinder.

The master cylinder is an important component of your car's braking system, responsible for turning the pressure from the brake pedal into hydraulic pressure that activates the brakes.

If the master cylinder fails, it can seriously compromise your car's ability to stop safely, putting you and your passengers in harm's way.

In this article, we'll talk about the common bad master cylinder symptoms and what you can do to address the issue. We'll also dive into why it's vital to tackle a bad master cylinder as soon as possible to ensure your safety on the road.

So, let's get started!

What Is a Master Cylinder?

A master cylinder is a hydraulic pump that takes the force you put on the brake pedal and turns it into hydraulic pressure that gets sent to the brakes to make them stop.

Without the master cylinder, your brakes simply wouldn't work. And if this component fails, you'll notice right away that something is off with your car's braking system.

If you're experiencing any of the symptoms discussed in this article, it's time to take your car to a mechanic to check out the master cylinder.

Bad Master Cylinder Symptoms

According to, there are a few signs to watch out for that could indicate your master cylinder needs attention. These bad master cylinder symptoms include:

1. Abnormal Brake Pedal Behavior

One of the most common symptoms is abnormal brake pedal behavior. The master cylinder is responsible for generating the pressure that makes your brakes work, so if it's having trouble sealing or distributing pressure, you might notice a change in how the pedal feels.

For example, it could feel mushy, spongy, or slowly sink to the floor when you press it.

2. Leaking Brake Fluid

Another sign to watch out for is leaking brake fluid. The master cylinder contains fluid that's crucial to creating the hydraulic pressure needed to slow your car down.

If you notice fluid leaking from the master cylinder or the Reservoirs that hold the fluid, your braking ability could be compromised. This is a dangerous situation that requires immediate attention from a mechanic.

3. Contaminated Brake Fluid

Contaminated brake fluid is another potential symptom of a bad master cylinder. Over time, the seals inside the cylinder can wear out and contaminate the fluid, which will turn it into a dark brown or black color.

When this happens, the cylinder may not be able to hold brake pressure effectively, leading to a mushy pedal or one that slowly sinks to the floor.

4. Check Engine Light

Finally, you might notice the check engine light coming on if you have a newer vehicle.

Some newer braking systems have sensors installed in the master cylinder to detect problems with the brake fluid pressure.

If the pressure drops, it could be due to a problem with the master cylinder, which could cause the Check Engine Light to come on.

What Makes the Master Cylinder Go Bad?

Several factors can cause a bad master cylinder, including:

  • Internal damage or wear: Over time, the internal components of the master cylinder can wear out or become damaged, leading to a loss of hydraulic pressure.
  • Contamination of the brake fluid: If the brake fluid becomes contaminated with dirt, debris, or moisture, it can damage the master cylinder's internal components.
  • Corrosion or rust: Corrosion or rust can damage the master cylinder's internal components, leading to a loss of hydraulic pressure.
  • Age or mileage: Like any other component of your car, the master cylinder can wear out over time, especially with high mileage.

How To Diagnose a Bad Master Cylinder

If you suspect that you have a bad master cylinder, it's important to take the necessary steps to diagnose the issue as soon as possible.

Ignoring a problem with your brakes can be dangerous, and a bad master cylinder can lead to compromised braking ability, making it critical to identify the problem and take corrective action.

According to our research, there are several steps you can take to diagnose a bad master cylinder.

The first step in diagnosing a bad master cylinder is to check the brake fluid level.

The master cylinder contains a reservoir that holds the brake fluid, and if the fluid level is low, it could be a sign that the master cylinder is failing.

Low brake fluid levels can also be an indication of a leak somewhere in the braking system, so it's important to check for any visible signs of fluid leaks.

Another sign of a bad master cylinder is a mushy or spongy brake pedal. If the pedal feels soft or spongy, it could be a sign that the master cylinder is failing to build up enough pressure to stop the car effectively.

A mushy pedal can also be caused by air in the brake lines or worn brake pads, so it's important to rule out these possibilities before replacing the master cylinder.

A third sign of a bad master cylinder is a brake pedal that slowly sinks to the floor when you press it.

This is a clear indication that the master cylinder is failing to hold pressure, which can compromise your ability to stop the car.

If you notice this symptom, it's critical to have your brakes inspected as soon as possible to determine the cause of the problem.

Finally, a bad master cylinder can cause your car to pull to one side when you brake.

This can be caused by uneven pressure distribution in the braking system, which can occur if the master cylinder fails.

If you notice your car pulling to one side when you brake, it's important to have your brakes inspected by a professional to identify the source of the problem.

How To Replace a Brake Master Cylinder

Replacing a brake master cylinder can seem like a daunting task for many drivers, but with the right tools and some patience, it's a manageable DIY project.

According to AxleAddict, a faulty brake master cylinder can cause serious problems with your vehicle's braking system, so you’ll want to replace it when you suspect it's not working properly.

Before you get started, gather the following tools:

  • Brake fluid
  • A new brake master cylinder
  • Wrenches
  • Pliers
  • A screwdriver
  • A vacuum pump

Now, let's walk through the steps to replace a brake master cylinder.

Step 1: Prepare Your Workspace.

Make sure your vehicle is on level ground and that the parking brake is engaged. Open the hood and locate the brake master cylinder.

Remove the brake fluid reservoir cap and use a turkey baster or similar tool to remove as much brake fluid as possible from the reservoir.

This will prevent brake fluid from spilling out when you disconnect the old master cylinder.

Step 2: Disconnect the Old Brake Master Cylinder.

Using wrenches and pliers, disconnect the brake lines from the old master cylinder.

Be careful not to damage the brake lines or fittings. Once the brake lines are disconnected, remove the bolts that hold the master cylinder in place.

Step 3: Install the New Brake Master Cylinder.

Carefully install the new master cylinder in place of the old one, using the bolts you removed in the previous step.

Reconnect the brake lines to the new master cylinder, making sure the fittings are tight and properly seated.

Step 4: Bleed the Brake System.

Bleeding the brake system is essential to ensure there is no air in the brake lines, which can cause your brakes to feel spongy or not work at all.

There are several methods to bleed the brake system, but one common method is to use a vacuum pump.

Attach the pump to the brake fluid reservoir and follow the instructions that come with the pump. Make sure to refill the brake fluid reservoir as needed during the bleeding process.

Step 5: Test the Brakes.

Once the brake system is bled, test the brakes to make sure they feel firm and responsive. If you have any doubts about the quality of your brake system, don’t hesitate to take your vehicle to a qualified mechanic.

Replacing a brake master cylinder can be a challenging DIY project. However, with the right tools and a bit of patience, it's a task that many DIYers can accomplish.

Remember to take your time, be careful not to damage any parts, and follow the instructions carefully. If you encounter any difficulties or are unsure of your ability to fix the issue, it's always best to seek professional help.


The master cylinder is a critical component of your car's braking system, and a bad master cylinder can lead to serious problems.

Look out for the bad master cylinder symptoms discussed in this article, and if you notice any, take your car to a qualified mechanic for diagnosis and repair.

It's essential to address a bad master cylinder promptly to ensure your safety on the road. Remember to have your braking system checked regularly to avoid any potential issues.