Top 6 Signs of Bad Brake Master Cylinder

How much do you trust your car brakes? You better be confident about having a healthy brake system when you’re rolling down the highway in your 2-ton car. But, let’s be honest, very rarely does anyone pays attention to brakes when sending the car for service. It’s mostly about engine maintenance.

In this post, we’re going to point out symptoms for a failing brake master cylinder. It’s a good idea to run through this list if you suspect braking issues with your vehicle.

Symptoms of failing Master Cylinder

1) Brake Fluid Leaking

Do you see drops of brake fluid under your car? Or is your brake fluid level keeps getting low? If your answer is yes to anyone of these questions, you might have a leaking master cylinder. The rubber seals in the master cylinder degrade over time and might fail one day.

The damage to the seals increases by not replacing brake fluid more often. It’s advised to
flush out brake fluid every 2 years. Using incompatible brake fluid could also damage the seals. Therefore, make sure you’re using the correct type of brake fluid. Most modern cars recommend DOT 4 brake fluid.

Before putting the blame on the master cylinder, it’s important to check if the brake fluid line itself is not punctured or has a loose connector somewhere. Fixing brake lines is a much easier job compared to master cylinder repair.

2) Spongy Brake Pedal Feel

Spongy brake feel is a sign of having air in the hydraulic brake system. In a well-maintained car, the brakes lines don’t have air trapped inside, and the brake pedal would feel hard while you step on it. This will make the car stop easily as pedal force can transfer to calipers efficiently.

If there is a leak in the system or if there’s air trapped, the brakes would feel springy or spongy. When you step on the brake pedal, it will sink in much farther with very little resistance.

We are feeling this effect as air is easier to compress but oil is much harder to compress. This is what enables it to clamp down the brake caliper on the rotors.

Air can get trapped in the system if the master cylinder is leaking, brake line is cracked or not connected securely, brake fluid is too low, or if the brake fluid has not been replaced for many years.

3) Sinking Brake Pedal

If the leak in the master cylinder is too severe, the brake pedal would just sink in fully when you step on it. When the pedal sinks in, you are essentially pushing out brake fluid out from the leaking area.

sinking brake pedal

A leak can also happen due to a loose brake line bolt, cracked brake line. But in most cases, it’s due to failed master cylinder or brake caliper seals.

4) Brake fluid level low

Although service technicians do check brake fluid level on each service interval. But even then, they rarely need to top it up. If the brake system is working well, the brake fluid should not need top-up even after 1-2 years.

So if you notice that the brake fluid is dropping every month, then that means there’s a leak in the braking system.

5) Dirty Brake Fluid

When the master cylinder seal or caliper piston seal fails, they can not only allow some brake fluid to leak out but can also allow dirt and debris to enter inside the brake system.

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This would make the brake fluid appear darker and even brownish in some cases. Usually, brake fluid is transparent or has a slight honey-like tint to it.

6) Uneven brake pad wear

For most cars, master cylinder has 2 separate brake circuits going out of it. Each circuit controls brake on two wheels. So if one circuit has some leak or blockage, the braking force will be less compared to other circuit that’s working fine.

uneven brake pad wear

This will result in uneven brake pad wear. Brake pads controlled by faulty circuit will have less wear.

You might not notice weak braking in day-to-day city commute. But, in case of emergency hard braking, the car will pull to one side.

Brake Master Cylinder Replacement Cost

The entire cost for replacing the brake master cylinder will land somewhere between $300 to $580, depending upon your vehicle. This includes labor cost along with part cost. This job generally takes around 2 hours to complete. Checkout price of master cylinder rebuild kit on Amazon

If you decide to do it yourself then you can save $200 to $300 on labor costs. Labor cost is a big part of this needs an experienced mechanic.

You can DIY this if you like wrenching and have some experience working on vehicles. Investing in a workshop manual for your vehicle is a good idea if you decide to work on it yourself.

If in doubt, paying the labor cost is worth it since braking performance should be a top priority when it comes to safety. Don’t forget to tell the mechanic to inspect degrading brake hoses and connectors also. Since this would be a good time to replace it as well.

After installing a new master cylinder, the brake system would need to be re-bled (the process of removing air from the brake lines). Keep in mind, some European cars required a computer hooked up into an OBD diagnostics port to perform brake bleeding.

How does Brake master cylinder work?

Both Disc and drum brake systems uses a master cylinder to clamp down the rotors or drum.

When you press the brake pedal, you are basically pushing down the master cylinder piston to pressurize the hydraulic system. When piston in the master cylinder is pushed down, it fills up pistons in brake calipers that squeezes the rotors.

The calipers on all four wheels are linked to the master cylinder via brake lines.

There are two types of master cylinders: Single channel and dual channel.

Single-channel master cylinder has one reservoir and one output feeding brakes of all four wheels.

Dual-channel master cylinder has two fluid reservoirs and two outputs, each controlling two wheels.

As you can imagine, dual-channel master cylinder is better in terms of safety. Even if one channel leaks or has some blockage, the other channel can still control brakes on the other two wheels.

How to prevent early failure of the master cylinder?

Aggressive, hard braking builds up lot of pressure inside the hydraulic system. And if you drive fast for a long time on a racetrack, the brake fluid can start to get really hot.

This not only builds pressure in the hydraulic system but could also break down and vapourize the brake fluid. This vapor will try to blow past the piston seals.

But in most cases, driving on public roads will not heat up the brake system too much. So for most drivers, changing the brake fluid periodically is the best thing they can do to prevent brake failure.

Fresh brake fluid has many additives that keep the seals conditioned. Over time, the brake fluid losses its additive pack and will get contaminated too. This old fluid can be harsh on the seals and can start to wear it off quicker.

Also read: How to eliminate brake grinding noise

Siddharth Sharma

Siddharth has always been passionate about Cars and Bikes. He was the kind of kid that always had the latest Auto magazine in his school bag. He had this dream- to become a professional racecar driver. Finally, in 2012 he found himself racing as a rookie driver in the Polo R Cup national racing championship. Over time he had to readjust the sail and get into automotive journalism to continue enjoying machines on wheels.