5 Signs of a Bad Brake Master Cylinder (& Replacement Cost)

master brake cylinder

Our vehicles must have working brakes to keep occupants safe, yet the braking system is made of so many working parts that can fail.

The brake master cylinder is one of the main components of the system, so when one goes bad, there are serious consequences to follow. 

However, a bad brake master cylinder causes some odd symptoms that aren’t always simple to diagnose. Here are five signs that something might be wrong with the master cylinder.

Bad Brake Master Cylinder Symptoms

  1. Strange Brake Pedal Behavior
  2. Inconsistent Brakes
  3. Decreased Braking Ability
  4. No Brakes
  5. Leaking Fluid

Here is a more detailed list of the most common symptoms of a bad brake master cylinder:

Strange Brake Pedal Behavior

push brake pedal

At first, you’ll likely notice the brake pedal acting unusually. The master cylinder generates all of the pressure needed to brake.

So, if there is a problem with distributing the pressure or sealing, you will feel it in the pedal. 

As the seals in the cylinder continue to wear out, there could also be leaks occurring. That’s part of the reason that the brake master cylinder causes a mush or spongy pedal. You might even have it traveling all the way to the floor. 

Sadly, this symptom alone won’t tell you that the master cylinder is bad. A spongy pedal more commonly means that there is air in the brake lines. Your car may just require a brake fluid change.

Inconsistent Brakes

brake pedal

As the master cylinder starts failing, the brakes can act erratically. At first, the brakes might work as normal, and the next second you could lose braking power completely. 

This is of course a very serious issue because it gets even more dangerous when you do not know when the brakes work as they should-

Additionally, the pedal can feel firm one second and head to the floor the next. 

Decreased Braking Ability

longer brake distance

When the master cylinder begins to fail, it can cause the brakes to go out in just the front or the back. When this happens, you will notice a significant decline in braking power.

As you prepare to stop, you aren’t counting on this extra time and you could end up getting into an accident. 

Other malfunctioning brake system parts can cause this same symptom. You might be dealing with air in the brake lines or old, worn-out fluid. These symptoms also occur if you have a blown brake line or hose.

No Brakes

flipped car accident

In the most serious cases, the brakes might not work at all. While this doesn’t happen often, it is possible. 

The master cylinder operates as a hydraulic pump, constantly pressurized the brake lines when you touch the pedal. With this design, it is much more common to lose brakes in either the front or back, but not both. 

However, you could find yourself with no brakes at all when the master cylinder fails. 

Leaking Fluid

car fluid leak

Inspect the system for any brake fluid leaks. If you notice leaking brake fluid coming down the back of the master cylinder along the brake booster or firewall, you have an issue.

You may even see brake fluid running down the firewall inside the cabin. Eventually, this leak will cause the system to run out of brake fluid, which reduces your stopping power.

As fluid continues to leak, you will also notice the spongy pedal we talked about earlier. While it might regain some firmness when you pump it, it won’t last for long until a repair is performed. 

RELATED: 5 Symptoms of a Brake Fluid Leak

The Function of a Brake Master Cylinder

brake master cylinder

The master cylinder acts as a hydraulic pump. It is responsible for feeding brake fluid to the brake circuit, where it converts the pressure from the brake pedal into stopping power. 

Think of the master cylinder as a syringe. Every time pressure is applied, the fluid pushes out of this cylinder to the brakes. 

Modern master cylinders contain two chambers, each operating one set of wheels. Because of this design, you should only lose brakes in the front or back when a failure occurs, not typically both. 

There is a reservoir built into the master cylinder that holds brake fluid. When the brakes are applied, fluid flows into the brake lines, which makes the brakes work. 

After the brake pedal is released, the fluid returns back to the reservoir, thereby releasing the braking system and allowing the wheel to move once again. 

Brake master Cylinder Location

brake master cylinder location

The brake master cylinder is located beneath the brake fluid reservoir. To find it, look under the hood on the driver’s side of the vehicle. 

It is typically found on the firewall. You will spot it because it has two or four brake lines attached to the main body. It’s also attached to the brake fluid reservoir with a small wiring connector. 

Brake Master Cylinder Replacement Cost

The brake master cylinder replacement cost is between $250 and $400. You have two options when replacing the master cylinder. You can choose a rebuilt cylinder or buy a new one.

Because of how important the braking system is to your safety, it’s often recommended to choose a new master cylinder. A new master cylinder costs $35 to $75, depending on your car model. The rest of the expense is the labor at your local shop. 

If you plan to do it yourself, you don’t need any specialized tools. However, you will want to factor in another $4 for the brake fluid and about $10 for the necessary bleeder kit.

Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. He has also been a motorsport (drifting) driver for over 5 years.