7 Reasons Why The Brake Pedal Goes To Floor When Engine Starts

My brake pedal goes to floor when engine starts, what can be the problem? It was rigid till then? And does this mean that my brakes won’t work when I drive? The answer is, unfortunately, yes. Read on to know what this means.

Brakes are the most essential component of your vehicle. They help in slowing or stopping down your car. If your brakes are not working properly, it is dangerous for you as well as others on the road. 

The NHTSA estimates that 2% of all car crashes happen due to brake failure. While that may not seem like a big number to you, that’s 2% of 2mn cars – which is about 40,000 cars every year (and these are 2005-07 stats, so the number could be much bigger now).

Brake Pedal Goes To Floor When Engine Starts

If you notice that your brake pedal is going to the floor during the starting of the engine, then you need to be extra careful. It may mean that your braking system has completely failed. I will recommend you not to drive your car if you see this happening.

There are numerous reasons for spongy brake pedals, such as:

  • Low brake fluid
  • A faulty master cylinder. 
  • Air may get trapped inside the brake lines

You need to find out the right cause for having a spongy brake pedal on your car and accordingly fix the issue. In this article, I will talk about more such reasons for the brake pedal going to the floor when the engine starts


You might like to read: Why Would A Brake Caliper Lock Up?


Why Does My Brake Pedal Sink To The Floor When Engine Running?

There are many potential reasons for brake pedals soft when engine running:

#1. Leakage Of Brake Fluid

The brakes in your car operate as a sealed hydraulic system which means the amount of brake fluid after the service should be equal till the time it needs to get replaced.

Brake fluid is a hydraulic fluid used to transfer the applied force into pressure. In other words, whenever you apply force on your brake pedal, this brake fluid converts this applied force into pressure, which stops your car.

The brake fluid needs to have specific characteristics to work effectively. For instance, they are always subjected to high temperatures in disc brakes or drum brakes. So it should have a high boiling point so that it will not vaporize from the brake lines. These brake fluids also protect the components like the master cylinder, piston and others.

But sometimes, the brake fluid level becomes low, which reduces the hydraulic pressure in the piston and brake pads. As a result, your brakes will become spongy or sink to the floor when the engine runs.

The brake fluid can become low when there is wear and tear in brake lines or if it has undergone rusting and pitting. It may also become bent during the accident and lead to brake fluid loss. Moreover, if there is an issue with the master cylinder, brake pads, or piston, brake fluid can also leak.

Common Signs of Brake Fluid Leakage

The brake fluid leaks are usually visible on the ground. If you notice a pool of fluid in your garage, i.e., most probably under the vehicle, it may be brake fluid, and you need to check the brake fluid reservoir immediately. 

Again, if there is a brake warning light on your dashboard, it will turn on when the brake fluid diminishes on the reservoir.


Here is what you need to do:

#1.1 Check The Brake Fluid Reservoir

You can lift the hood and check the brake fluid reservoir. The fluid reservoir looks like a small plastic bottle with a small pipe below it and has markings on the side of the container. 

The top one indicates the maximum amount of brake fluid in the reservoir, and the bottom one indicates the minimum amount of brake fluid.

If you find the brake fluid has dropped just below the minimum ten, fill the reservoir with brake fluid. But if you find the reservoir empty, then the brake system requires bleeding.

#1.2 Inspect Brake Lines

You need to inspect the brake lines of your car thoroughly. Wear a rubber glove and rub your hand on the brake lines. If you find any oily residue, there is a problem with brake lines.

#1.3 Inspect Caliper

Sometimes, the brake caliper and piston fail and drain all the brake fluid completely. So, you need to inspect the brake caliper and piston on the wheel.

Brake Pedal Goes To Floor When Engine Starts

#2. Master Cylinder Failure

Failure of the master cylinder can be the reason for the brake pedal sinking to the floor when the engine is running.

The master cylinder plays a vital role in generating hydraulic pressure. Cars usually have one master cylinder for four wheels, but in modern cars, you will find two master cylinders, one for the front and the other for the rear wheels.

It is usually sealed and thus sends the pressurized brake fluid to the pistons in the caliper so that it stops the vehicle. But if the rubber seals undergo wear and tear, it can lead to internal braking, and consequently, your brake pedal goes to the floor when the car is running.



The rubber seals wear with time. So, you need to check the rubber seals of the master cylinder from time. If you find any wear and tear, then replace the master cylinder with a new one. Don’t install a used one because it will work well for a short time, and then it will have problems.

#3.Worn Brake Pads

A thin or worn-out brake pad can cause no brake pressure when the car is running.

Brake pads play a vital role in stopping your vehicle. It is an essential component of disc brakes. They consist of steel plates with friction material that faces the rotor of the disc brakes.

These brake pads are rubbed against the rotor whenever you engage your brakes, which causes a minute amount of wear and tear. The deterioration adds up every time, and consequently, it becomes thinner and thinner, and a time comes when it needs to be replaced.

The brake pads usually last for 30,000 to 35,000 miles. However, it depends on the road where you are driving and how you are driving. For instance, if you are driving on an urban road, you engage the brakes more frequently than on a rural road. So, your brake pads wear out quickly.

Again, some people habitually engage the brakes more frequently than others. It may also lead to wear and tear of brake pads quickly.


You need to inspect the brake pads from time to time visually, and if you find it’s thinner or worn out, then replace them with a new one.


Some cars have an indicator light on their dashboard, which signals the driver to replace the brake pads. You need to check the manual to determine whether your vehicle is equipped with a thin pad warning system.


You might like to read: Can’t Build Pressure When Bleeding Brakes – What to Do?


#4. Air In Brake Lines

The braking system in your car is designed to be air-tight, but brake fluids are usually hygroscopic. Air may enter inside the fluid during repair and retain inside it. Air can also get inside if you are driving with worn-out pistons or calipers.


Brake fluids also have an extremely high boiling point. So, when you drive your car in mountain or hilly regions, the brake fluid heats up to the boiling point temperature or may go beyond it. As a result, the water inside the brake fluid gets heated up and convert into steam.


Now, what is the problem with air in the brake lines? The hydraulic brake fluid is incompressible. So it transfers force from the pedal to the brake caliper and thus helps stop the vehicle. On the other hand, if air can be easily compressible. So, it absorbs the force instead of transferring it to the brake caliper. As a result, you will experience a soft brake pedal when the engine is running.


The only solution to remove air from the brake lines is to bleed the brake fluid.

#5. Faulty Brake Caliper

The brake caliper plays a vital role in the disc braking system. The pressurized brake fluid from the master cylinder enters the caliper. It exerts pressure on the piston and also uniformly in all directions. 

Consequently, the brake pads come in contact with the disc or rotor, stopping your car. But when the pistons get wear and tear or undergo rust, the brake pads cannot contact the rotor. So there will be no brake pressure when the car is running, which is dangerous.


Check your brake caliper from time to time. If it has undergone a lot of wear and tear, you need to replace it with a new one.

#6.Faulty Brake Booster

You may have a faulty brake booster when the brake pedal is soft when the engine is running.

Whenever your vehicle is heavy, you need to apply more force on the pedal, which is quite tricky for you. But in modern cars, a vacuum booster simplifies your work.

The brake booster is also known as a vacuum booster. It is usually located in between the master cylinder and the brake pedal and uses a vacuum to multiply the force applied on the pedal.

The vacuum booster has a diaphragm in the center and has two chambers, i.e., one at the front and the other at the rear. When you don’t press the brake pedal, both sides of the chamber have a vacuum. 

But when you press the pedal, the shaft moves forward and opens the valve at the rear side of the chamber, But the other side is still filled with vacuum. 

There is a difference in pressure created between the two chambers of the diaphragm by which the shaft presses the piston rod of the master cylinder, which multiplies the force you apply to the pedal. 

It makes your car’s stopping easier, and you can drive safely and comfortably. In some cases, the brake booster can add 200-300lbs. However, it depends on the type of diaphragm, atmospheric pressure in the air, and the amount of vacuum in one diaphragm chamber.

When you release the pedal, the spring makes the shaft return to its original position. The vacuum also becomes equalized on both sides of the chamber.

But whenever there are faults in your brake booster, the stopping distance increases. The brake pedal sinks to the floor when the engine is running.


You have to replace the brake booster with a new one.

Brake Pedal Goes To Floor When Engine Starts

#7. Faulty Wheel Cylinder

In some cars, you will find two braking systems, i.e., disc brake and drum brake.

Drum brakes are a braking system in which a brake drum or rotor rotates with the wheel. There are brake shoes, brake lining, and the wheel cylinder or slave cylinder inside the brake drum.

When you press the brake pedal, the pressurized brake fluid from the master cylinder enters the slave cylinder. The pressurized fluid put pressure on the pistons of the slave cylinder. 

As a result, the piston expands and presses the brake shoes and brake lining. The brake lining comes in contact with the rotating drum and stops the vehicle.

But when the pistons in the wheel cylinder undergo wear and tear, there is a leakage of brake fluid inside the rotating drum. It increases your stopping distance, and the brake pedal goes to the floor when the engine is running.


If your mechanic detects a problem with the wheel cylinder, you may need to replace it with a new one.

Do Brakes Work When The Car Is Off?

Yes, the brakes will work when the engine is turned off, but it will not work the same way as in normal driving conditions. You can stop the vehicle, but the pressure required to stop your car comes directly from you and not the vacuum booster. Hence, it takes a lot more effort from you to stop the car.

Brake Pedal Goes To Floor When Engine Starts

A Few Final Words

You should give top priority to the braking system of your vehicle. If you notice that your brake pedal is sinking to the floor, then stop the car right away and find the root cause for such an issue. 

If you are confused, then you better take your vehicle to the nearest mechanic. He will find the primary reason for the brake pedal sinking to the floor and repair it accordingly. Thank you for reading the article, we hope we have answered your questions about faulty braking.

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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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