How to spot a bad brake booster or 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.



The symptoms of a bad brake booster and master cylinder are hard to ignore. If you notice any or all of the symptoms mentioned above, take the car to a garage or reputable mechanic to fix the problem. Make sure you’re running some good brake fluid and your brake pads are still in good shape.

Do You Have A Hunch That Your Master Cylinder Is Bad?

If you suspect that your master cylinder is bad, you don’t have to bring your truck to a shop to confirm it. There are 3 easy things you can do to confirm the issue. Be sure to do all 3 things rather than just 1 or 2.

1. The Brake Pedal Test

The brake pedal test will help you determine whether the master cylinder is the cause of the symptoms you’re experiencing. Do this with the engine running while parked:

  1. Press down firmly on the brake pedal until it comes to a stop.
  2. Hold the pedal down.
  3. Observe whether the pedal sinks or not.

If the pedal begins to sink slowly, the master cylinder has an internal or external leak.

This is a good way to see if the master cylinder is the issue. You still want to fully confirm the issue, though. That’s why we have 2 more steps for you to complete.

2. Checking The Brake Fluid

If the rubber seals in the master cylinder wear out enough, they can contaminate the brake fluid. Brake fluid should be clear with a hint of yellow. If the brake fluid is dark (as in black or brown), that means it absorbed a lot of debris. The debris is likely from the worn rubber seals.

You also want to make sure that brake fluid isn’t leaking out of the master cylinder. You can do this by checking the brake fluid level in the reservoir.

To check your brake fluid reservoir:

  1. Open the hood.
  2. Find the brake fluid reservoir. In most trucks, it’s mounted on or near the firewall in the rear of the engine bay. If you have trouble finding the brake fluid reservoir, check your owner’s manual.
  3. Check the brake fluid level. There should be a line inside the reservoir marked “full”. If the fluid level is too low, it may mean that there’s a leak in the master cylinder.
  4. Check the brake fluid color. Is it dark? If so, it’s likely that the rubber seals in the master cylinder are too worn.

3. Inspecting The Master Cylinder

Now it’s time to visually inspect the master cylinder. To do this:

  1. Locate the master cylinder. It’s right underneath the brake fluid reservoir.
  2. Using a flashlight, thoroughly inspect the master cylinder. Look for any:
    • Signs of leakage. Look for oily dirt stuck to the master cylinder and check all the connections to brake lines for leaks, too.
    • Cracks
    • Loose fittings

If you find any physical defects, the master cylinder absolutely needs to be replaced.

Next Step

Schedule Brake System Inspection

The most popular service booked by readers of this article is Brake System Inspection. Once the problem has been diagnosed, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. YourMechanic’s technicians bring the dealership to you by performing this job at your home or office 7-days a week between 7AM-9PM. We currently cover over 2,000 cities and have 100k+ 5-star reviews… LEARN MORESEE PRICING & SCHEDULING

3. Brake Fluid Leakage

If you notice any leakage from the brake master cylinder, it may indicate that the cylinder has problems. For the cylinder to work well, it needs an adequate amount of brake fluid to exert enough pressure to stop the car. When the fluid leaks, you'll find it difficult to stop the vehicle.


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.

How To Track Down A Brake Fluid Leak

Symptoms of a brake fluid leak can be gradual or sudden. The brake pedal may feel mushy or drop to the floor, and you may see a brake warning light. If you suspect there is a leak in the brake system, it’s important to…

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Signs It’s Time To Replace Or Repair Your Master Cylinder

Brake Warning Light Illuminates

If your vehicle is equipped with brake fluid or pressure sensors, then it will likely give off an alert if either of them drops too low. It will do so via either a specific “brake light” indicator or a check engine light.

If it lights up a brake light indicator, then you at least know which system is having an issue. If it only illuminates a check engine light, then you’ll have to use an OBD2 sensor or take it to a shop to diagnose the exact cause.

You have to remember that the master cylinder is mandatory for your braking system to function. For this reason, be sure to tackle the issue before it leads to something worse.

Brake Pedal Sinks Or Feels Spongy

Here’s an easy one to notice. If you step on the pedal and it feels squishy, spongey, or less responsive, there’s likely an issue with the master cylinder. The brake pedal may also sink, meaning it doesn’t return to its regular position.

This usually happens due to a problem with the seals inside. Their purpose is to keep the fluid in so it doesn’t leak. If they’re worn or damaged, the fluid will seep out. When this happens, the calipers won’t receive enough to adequately squeeze the rotors and slow your vehicle.

Brake Fluid Is Low

There are three reasons you’ll find your car low on brake fluid.

The first is forgetting to add more, which should be done every 2-years or so. The second is similar to the last symptom – a leak due to a worn or damaged seal. The third is that the reservoir connection is unsecured, meaning fluid will seep out before reaching the master cylinder.

The last thing you want is total brake failure, which can be disastrous. Instead, when in doubt, check your fluid levels.

To do this, simply locate the master cylinder cap on top of the reservoir and make sure it’s not low by reading the levels on the side. It’s usually found in front of the brake pedal near the firewall between the cab and the engine.

Contaminants In The Brake Fluid

Here’s another one that stems from worn or damaged seals – contaminants in the brake fluid. These seals don’t just ensure fluid stays in, they also keep things like dirt, rust, and dust from getting in.

This usually coincides with a less responsive brake pedal, meaning you’ll have to press harder to get the same results.

You can also visually inspect the fluid to ensure it’s the correct color. It should be clear, perhaps with a slight yellow tint. If it’s dark brown, or turning black, then your brake fluid is contaminated. You might also notice gunk on the bottom of the master cylinder cap.

Brake master Cylinder Location

The brake master cylinder is located beneath the b

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. 

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