6 Symptoms of A Bad Master Cylinder

Do you know how to tell if the master cylinder is bad? The master cylinder is an inevitable component of your brake system. Your car brake system cannot function when you have a bad master cylinder. It is essential to convert mechanical energy applied to the brakes when you want to stop your car to the brake clippers’ hydraulic pressure to pressure the brake pads, thus stopping your car.

With all this importance a brake cylinder holds in your car, it can sometimes fail. That scenario is very detrimental and risky to your life and the people driving next to you. It would be best to diagnose the problem early to ensure your master cylinder remains active to keep you safe.

It is good to know how it works to understand the brake master cylinder problems. Here, you will find six bad master cylinder symptoms and a detailed explanation of everything you ought to know about the master cylinder. Firstly, read through the components master cylinder and understand its function. From understanding its work, you can diagnose the signs of a bad master cylinder.

How Master Cylinder Works to Stop Your Moving Car

The brake system has several components, interlinked and dependent on each other for its proper functioning. The master break is essential since it converts energy applied on the pedal by the driver’s foot. This energy should travel to the brake pads but cannot work without being converted to hydraulic pressure for the brake fluid.

It converts the mechanical energy to hydraulic pressure by forcing it to the brake fluid. The pressure, in turn, goes into the brake circuit to execute the stopping of the car. The master cylinder also regulates the amount of brake fluid sent to the circuit depending on the pressure you apply on the pedals. You will find a master cylinder with drum brakes or disk brakes.

Also, cars should have two brake circuits. There is a tandem master cylinder that generates the pressure required by the circuit. Through this, one brake system is responsible for stopping the car, while the other ensures the circuit remains functional if it fails.

How To Tell If the Master Cylinder Is Bad

These are master cylinder symptoms that show it has failed and gone bad. There are serious cases that may need replacement. Upon noticing the highlighted symptoms, you should immediately check and call your mechanic to ensure your safety on the road.

1. Warning Light

Fragment of instrument panel of car speedometer, tachometer warning light

The warning light is the first symptom that you can easily notice on your dashboard. The brake light will illuminate to issue a warning. The warning always implies there is something wrong with your brake system and not necessarily the master cylinder. Anytime pressure is applied on the pedal, and the master cylinder will produce pressure to push the piston in the cylinders. These movements are distributed equally to all brake components to aid in stopping your car.

The brake fluid is responsible for the transmission of the pressure throughout the break systems. Beneath the reservoir that hosts the brake fluid, a sensor is connected to the car’s dashboard. Its primary function is to detect if the amount of brake fluid is enough at all times. When there is little brake fluid, a light signal is sent to the dashboard to alert you of the issue.

However, sometimes it should not be a cause for alarm. The sensor can also malfunction and send alerts that are not true. Ensure your mechanic checks it well when this happens. Your master cylinder may also be the only cause of the light, hence the need to take immediate precautions.

2. Spongy or Abnormal Brake Pedal

The master cylinder is responsible for the functioning of the brake system. Once it gets faulty, the effect can be felt when you press the brake pedal. The master cylinder fails to transmit pressure and or has a problem with the sealing. With tear and wear, the seals on the master cylinder will reduce and cause leaks.

As such, your brake pedal will feel mushy, spongy, or slow when you try to press on it. Most brake pedals are firm when pressed, and that should always be the case.

When the master cylinder cannot convert the pedal’s mechanical pressure, it feels spongy since there is no activity generated by the action. Anytime you feel an abnormal behavior with the pedal, high chances are the master pedal has bad seals. The spongy brake pedal is also a symptom of air in the master cylinder.

It will produce a spongy feel due to the air bubbles trapped between the brake fluid, causing it to flow abnormally. Also, the damage to the brake lines and rust can cause a spongy feeling.

3. Uneven Brake Pad Wear

When the circuit fails, the pressure will not be equally distributed to the four wheels of your car as intended. You will realize the pad wear is uneven, and only two of the four wheels stop with the pressure applied. The pads also wear unevenly when the master cylinder is faulty. It implies an issue with the piston seals on the master cylinder hence the failure to distribute pressure evenly. It can also happen when the brake line is damaged and there is a leak. It will cause malfunctioning of one side of the circuit.

Some cases resulting from such a faulty master cylinder include:

  • Uneven braking
  • Uneven pad wear
  • Car pulling to one side when braking

4. Brake Fluid Leaks

Brake fluid topping up

When your brake fluid level drops, it may be due to a leak from the brake lines and the master cylinder. The master cylinder is below the reservoir of the brake fluid. Leaks can form from the valves in the brake circuits or the fluid control valve that takes the fluid to the reservoir and down to the master cylinder.

Damaged brake lines can cause leaks through tear and wear or rust. You can see the brake lines’ signs when you peep on the brake lines under your car.

5. Contaminated Brake Fluid

There is a rubber seal on the master cylinder. When damaged, the rubber seals can cause brake pedals’ spongy feel and lead to brake fluid contamination. The seal is supposed to seal efficiently to prevent materials such as dirt, water, debris, and any foreign particle that should not be in the brake fluid.

These contaminants will mix with the fluid and make it very hard to stop your car since it will mess up with the fluid’s hydraulic pressure. Sometimes it takes very long to stop your car as a result of a contaminant in the fluid. The pressure takes time to act on the wheels to stop them.

When this happens, it would be best to get your mechanic to examine your brake circuit, the brake line, the master cylinder, and your brake pads.

6. Sinking Brake Pedal

Typically, the brake pedal remains firm and works appropriately by moving down and up in a graceful motion when you step on it. A bad master cylinder will result in a sinking brake pedal when pressed. It is a specific test to prove your master cylinder is faulty.
The brake pedal is flexible, and when released, it should return to the original position instantly.

A skinning pedal will remain where you left it after pressing it. It will stay close to the floor, and this is worse since you need to brake often. Some may come back to the original position, but it takes time, making it very unreliable. Driving a car with a sinking brake is very dangerous. You are always at risk since you cannot make sharp brakes. Ensure your brake is checked immediately this occurs.

These are the symptoms of a bad brake booster. Here is a guide to help you test the brake master cylinder problems.

Simple Steps to Evaluate the State of Your Brake Booster

You can use these steps when you are alone and want to check your master cylinder’s health. Use them effectively to ensure your safety on the road always.

Park your car on a level ground

Your car should not roll away during the exercise. Ensure the hand brake is engaged.

Pop the hood and look under it

Check the brake lines for rust or leaks. They must be replaced as soon as possible when they show weakness.

Start your car to run your engine.

Make your engine idle for about 3-5 minutes as it runs smoothly. Spray the vacuum hose with soapy water to check for any bubbles. Bubbles will show you some leaks on the tube.

Car engine


1. Can I drive with a bad brake booster?

No. It is hazardous for your passengers and other road users. Ensure the problem is fixed before you drive your car.

2. Why is my pedal feeling spongy?

The spongy feel can be a result of a bad master cylinder. You should check your brake fluid for foreign contaminants such as air or brake fluid leak.

3. How do I fix a sinking brake pedal?

Mostly brake pedals sink due to faulty master cylinder. It can be very severe to make your pedal stay on the floor after you remove your leg. Ensure you get a quality mechanic to work on that.

4. Will air in my brake lines go away?

No, unfortunately, the problem has to be fixed by a professional mechanic. If you leave it for long, the problem may persist and result in more damage.


A master cylinder is essential. You cannot travel with a faulty brake system. Ensure your brakes are always efficient by checking on these signs to prevent master brake cylinder failure and keep safe on the road.