Spongy Brake Pedal When Engine Running – What to Do?

When your brake pedal doesn’t have adequate pressure, it becomes dangerous. Let’s discuss the reasons for a spongy brake pedal when engine running in detail.

Brakes are a vital component of your vehicle. A proper braking system in your four-wheel vehicle ensures that your car stops at the appropriate time. 

If your brakes are worn out or not functioning properly, your car may not stop as quickly as you want. This can be extremely dangerous, especially if you are driving fast. You might end up hurting yourself and other cars or pedestrians.

The brake pedal is very sensitive. It should feel firm when you are driving your car. But if it goes too down and touches the floor on applying even very little pressure, it is a red alert that your brakes are not functioning properly. 

Your first responsibility should be to ensure that you fix your brakes as soon as possible.

In this article, I will discuss various causes of the spongy brake pedal when engine running and other braking system issues in detail, which will help you diagnose the braking problem so that you fix it accordingly.


Spongy Brake Pedal When Engine Running

Why Is My Brake Pedal Spongy Or Sinking To The Floor When The Engine Is Running?

Soft brake pedals when car is running at high speed can be disastrous. Below, I will list some of the reasons why this might be happening.

#1. Brake Fluid Loss

Brake fluid is a hydraulic fluid that plays a vital role in stopping the brakes. It converts the energy which you apply to the pedal into the force that stops your car.

In disc brakes, you will find a piston inside the caliper. Brake pedals are attached to the piston. When you apply the brakes, the pressurized brake fluid in the master cylinder puts pressure on the piston.

The piston pushes the brake pads, which come in contact with the car’s wheels, and that is what finally stops your vehicle. Now think about it: if there is no brake fluid, how can the brake pedal exert sufficient pressure on the piston? 

This is why when there is a leak in brake fluid; you get a spongy brake pedal. Your brake pedal will sink to the floor immediately without slowing down the car at all. 


You will have to first determine the source of the leak and ensure that it is properly plugged. After you carry out the necessary repairs, simply bleed out the old brake fluid and air from your braking system and replace it with new fluid.


Spongy Brake Pedal When Engine Running

Also read: Meaning of the sign “No Engine Brake”


#2. Master Cylinder

The master cylinder is responsible for transferring the pressure of the braking fluid to the brake calipers. So, the rubber seals which help in keeping the brake fluid inside should not be damaged or worn out. 

If they get damaged, the brake fluid may leak internally, which will lead to a soft brake pedal when the engine is running, as explained in the previous point. So, don’t take this problem lightly.


To fix this problem, you first need to check the fluid reservoir of the master cylinder. If the reservoir is empty, you will know that brake fluid is leaking somewhere, and you have to solve the problem accordingly. 

But if it is low, it may be because some of the components have frozen or got rust and need replacement. Sometimes you can fill the brake fluid again and notice the changes in the brake pedal. 

If you still notice the spongy brake pedal when the car is running, you need to find out the reason.

#3. Air In Brake Lines

The brake fluid is hygroscopic by nature which means it can readily absorb water. But after absorbing water, the boiling point of brake fluid decreases. Consequently, brake fluid can boil at a fast rate and convert into steam. This steam leaves behind air in the brake lines.

Air in the brake lines is one of the biggest causes of spongy brakes. Brake fluid without air exerts pressure uniformly on the piston of the brake caliper. But when there is air, the pressure is not distributed uniformly, which makes the brake pedal go to the floor when the engine starts.

Sometimes your brake pedal will sink to the floor after you change your brakes’ master cylinder, which may happen because there is air trapped in the brake lines when you install the new cylinder.


Spongy Brake Pedal When Engine Running


If you are facing spongy brakes just because of air in the brake lines, I suggest you flush out the brake fluid. This is also known as bleeding of brakes to eradicate air from the brake lines. 

The clean and new brake fluid will prevent the brakes from becoming spongy and protect other brake components like the master cylinder so that it will last for a more extended period.

#4. Brake Line Damage

When your brake cables are damaged, then it takes a lot of effort to stop your car. The brake lines or cables may rust due to salts on the road and moisture. 

Sometimes the brake lines get bent or damaged because of a car crash, which leads to brake fluid leakage, which is the primary cause of spongy brakes.


You need to sand the brake lines and remove the rust. You should also ensure to clean the brake lines regularly to avoid road salts accumulating on your brake lines.

If the brake lines are bent or damaged, you should get them replaced immediately.


Spongy Brake Pedal When Engine Running


#5. Faulty Disc Brake Calliper

Your car’s braking system has to deal with a large amount of heat to stop your vehicle. The brake calipers may get damaged because of constant exposure to heat, or they may get rust over time. 

The pistons seals inside the caliper may corrode and lead to internal brake fluid leakage. Consequently, it leads to a soft brake pedal when the car is running.


Typically, this would require a trip to your nearest car garage. 

#6. Faulty Brake Booster

If your brake pedal goes to the floor when the engine starts, then it may be because of a faulty brake booster. 

Think about it: your car is pretty heavy, so you would ideally need to apply an equivalent amount of force to stop it. Then how do you get away by simply pushing your brake pedals?

The brake booster amplifies your brake’s capacity to bring the car to a stop, so you don’t have to press the brake pedal too hard.

The brake booster is usually present in between the master cylinder and brake pedal. But when there is a problem with the brake booster, you may have to put a lot of effort into stopping the vehicle. 

The brake pedals become spongy and sink to the floor to stop the car completely, indicating you are not getting enough power to prevent the car.

Sometimes, you may notice that your brake pedal becomes too hard after starting the engine, which may also be because of some faults in the brake booster, which happens because of damage to the vacuum diaphragm present inside the brake booster.


Spongy Brake Pedal When Engine Running


  • Sit on the seat of the driver but down run the engine. Now press the brake pedal three times to release all the pressure. You will notice that the brake pedal is difficult to press down.
  • After some time, start the engine and again press the brake pedal. Now it will be down to some inches, which proves that your brake booster is working perfectly.
  • After starting the engine, if your brake pedal does not drop, there is a problem with the brake booster.
  • Sometimes the vacuum system of your brake booster has a problem, and if the problem is major, then it’s better to replace the brake booster with a new one.

#7. Leaking Slave Cylinder Or Wheel Cylinder

The slave cylinder in drum brakes is also known as a wheel cylinder. In some vehicles, you will notice disc brakes on the front and drum brakes on the rear wheel.

In drum brakes, you find two cylinders. One is a master cylinder, and the other is a slave cylinder. These brake systems have a drum that rotates along with the wheel. 

The drum has a slave cylinder and brake shoes inside it. Whenever you apply brakes, the brake fluid from the master cylinder flows to the slave cylinder and applies pressure on the piston of the slave cylinder. 

As a result, the piston pushes the brake shoes. Finally, the brake lining comes in close contact with the drum, which results in retardation of the vehicle.

But if there is corrosion or rusting inside the slave cylinder, that may lead to leakage of brake fluid, resulting in spongy brakes.


Spongy Brake Pedal When Engine Running

Frequently Asked Questions

#1. Why does my brake pedal go to the floor when I start my car?

The most common reason behind this is leakage of brake fluid. There are other reasons, too, like if there is a problem in the master cylinder or wheel cylinder, your brake pedal may also sink to the floor.

#2. What is the most common cause of spongy brake pedals?

Air in the brake cables is one of the significant causes of the spongy brake pedal. The braking system in your vehicle depends on equally distributed hydraulic pressure to bring it to rest. But as there is air inside the brake fluid, it causes an imbalance in pressure, resulting in spongy brakes.

#3. Can a bad brake booster cause a spongy pedal?

The brake booster provides power to your braking system. If it is faulty, then you may notice spongy brakes, or your brake will not operate.

#4. How do I know if my brake master cylinder is bad?

You can diagnose the bad or faulty master cylinder if you observe the following conditions

  • When your car brakes do not work properly
  • Spongy brakes or your brake will sink to the floor
  • Brake fluid leakage


Spongy Brake Pedal When Engine Running

Spongy Brakes Can Happen Due To Many Reasons. Regular Maintenance Is The Only Way To Avoid It.

Your brakes in the car should always be in good condition, which ensures your safety while driving. Well, sometimes, you may notice the brake pedal sinking to the floor. That may be because of the above reasons. 

So if you see such things, it is better not to drive your car. Instead, take your vehicle to the nearby car garage. They will help to diagnose the problem, and your brakes will repair within less time.

I hope you liked my article, and don’t forget to share your suggestions in the comment section.


Happy Driving!

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Sean Mendez

Hi, I am Sean, a self-confessed petrolhead. I live in Boise, Idaho with a busy family of four and our energetic Labrador retriever. Thank you for visiting my website. You can find my email on the contact page.

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