Idle Drops When Brake is Pressed (What to do)

We get scared when we see our engine idle drop when the brake is pressed. Wondering what went wrong though you cared about your car so much.

Why do idle drops when brake is pressed?

Idle drops because of the bad brake booster. It is one of the most common issues. A faulty master cylinder can be another reason. The idle drops if the throttle is defective or not operating properly. Lastly, a faulty oxygen sensor causes idle dropping. These problems can be fixed with easy solutions.

There’s nothing to be worried about. Just follow the details and notice what is the reason causing the issue. Read-Along!

Why the Idle Drops Why the Brake Is Pressed- With Solutions 

So, you noticed engine idle changes when brakes were applied. You need to immediately check up and detect the actual reason behind idle dropping. 

Let’s find out all the possible problems with step-by-step solutions.

Reason 1: Bad Brake Booster

The brake booster should be the first item to be checked if your idle drops when the brake is pressed. It is essential to the functioning of the braking system. You will see idle drops when put in drive to somewhere. 

Having a leak in the brake booster causes more air to get into the engine’s intake manifold. Consequently, the idle drops more as a consequence. 

Brake booster leaks may also cause the brake pedal to sink, braking pressure to decrease, and engine misfires to occur.


It’s best to leave the repair of a brake booster to a trained technician. The brake booster adjustment may still be done by you. That is if you have the proper tools and know-how.

Tools like screwdrivers, different-sized wrenches, and a new check valve are needed initially.

  • Start by turning off your car’s engine. Use a hand pump to remove as much brake fluid as possible from the system. 
  • Remove the clamps and remove the valve line using a screwdriver or wrench after identifying the valve.

Remove the check valve and install a new one if you feel it is defective. Reconnecting the suction hose should reveal whether the problem has been resolved.

As long as the brake booster pressure is enough, you’re set to go.

  • Retrace your steps back to where you found the brake booster. The brake master cylinder’s firewall is where you’ll often find it. 

Disconnect the vacuum line and clamps next. Then, disconnect the brake booster from the master cylinder and remove it from the vehicle.

  • Replace the old brake booster with the new one. Set in place the new brake booster. Reconnect the vacuum hose and clamps to the booster. Then, slide the rubber hose over the fittings.

That’s all!

Reason 2: A Faulty Master Cylinder

The master cylinder and the brake booster function together. Both of these increase the amount of pressure you exert on the brakes. 

As a result, when you use the brakes, your idle will drop. Also, are there worn-out seals on the Master Cylinder or strange clutch pedal action? They are all symptoms of damaged master cylinders.


If you have the necessary tools, you can easily replace a master cylinder. Step-by-step instructions for replacing a master cylinder that has worn out are provided in this article. 

  • A brake master cylinder that has failed is located. This is why when you start to hit brakes and idle drops. 

Typically, the master cylinder is positioned underneath the brake fluid reservoir. The brake master cylinder should be attached to the lines that carry the braking fluid. 

  • Using a wrench, remove the master cylinder. Use a wrench to loosen and remove all three nuts holding the master cylinder in place. A line wrench may also be used to free the brake lines from the master cylinder ports. 
  • Next, unplug the fluid lines’ electrical connections and any other fittings. Using the new master cylinder, attach the fittings to it. The fluid reservoir may need to be transferred to the new cylinder, depending on the vehicle type. 
  • Refill the new brake master cylinder with antifreeze. In order to remove any remaining air from the system, it is necessary to perform a master cylinder bleed. 
  • Install the brake master cylinder once it has been bled and reassembled. Tighten the bolts on all the brake lines. Using a line wrench, further tighten the hoses. 

The reservoir should be topped up to complete the installation. Your idle will drop when you brake if you properly install the brake master cylinder. 

There’s more. Read along!

Reason 3: Troubleshooting of Throttle 

When braking, the idle drops if the throttle is defective or not operating properly. 

It is possible that the throttle may get stuck and not shut completely. As a consequence, while parking, the vehicle’s engine RPM drops when stopping. 

When you stop your car, the engine should return to idle if the throttle is operating properly. You need a throttle to regulate air movement in and out of the cylinders. This is used to consume fuel in them. 


If the old or worn-out throttle body still works, consider cleaning it first before replacing it. If not, you may just buy a new throttle body and install it. 

  • Find the throttle body first. It will be a part of the carburetor if your automobile has a classic carbureted engine. 

A contemporary fuel-injected engine will have the throttle body positioned between the air filter and the intake manifold. This is more common when idle drops troubleshooting happens. 

  • Remove the air duct by turning off the engine. It’s important to keep an eye out for any indications of wear on the throttle body. 
  • Remove all the hoses from the throttle and disconnect the throttle wires. Remove the screws holding the throttle body as well. 
  • Disconnect everything and then remove the throttle body. Disinfect and reassemble all of the components and ports. 
  • Remove any carbon buildup from the components. Check the new throttle body to see whether it’s compatible with the old one. Fasten the screws after putting them in place. Make sure all the detached pieces are reconnected. 
  • Test your new throttle body after you’ve completed the connection. You may not need to replace the complete throttle body in certain circumstances. 

You may quickly fix an idle drops issue if the cause is a problem with the throttle body pressure. Adjust the throttle stop to ensure that your engine is running at its peak performance.

Reason 4: A faulty oxygen sensor 

The oxygen sensor is another component that’s easy to ignore yet has the potential to create issues with your vehicle. 

An engine control unit (ECU) may alter the amount of air and fuel in a car’s engine by using this component. When you brake, an issue with the oxygen sensor might cause the idle dropping issue.

In addition, it may cause a poor gas economy and a failed emission test. A sluggish engine idle, and a check engine light to turn on also caused the issue. The oxygen sensor thinks you have too much gasoline in your air-fuel combination. 

You should replace your oxygen sensor every 60 000 to 90 000 kilometers. If you’ve driven more than this amount of miles, it’s time to get a new oxygen sensor.

If you find out a faulty oxygen sensor, replace it immediately with the provided good quality one.


You may clean and reinstall the sensor if it is filthy. A broken one means that you’ll have to get a new one. Techniques like these should be used: 

  • Locate the faulty sensor in your vehicle. To get the work done, you just need the necessary equipment and procedures. Remove the faulty sensor once it has been tested and identified.
  • Use a socket set to remove all the retaining bolts. Removing the sensor from the electrical wire harness is a simple matter of doing.
  • Use a wire brush to clean the sensor’s metal tube end to remove any remaining dirt and grime. Spray and clean it with compressed air. 

In the event that this is not feasible, put some gasoline in a container. Put half the metal tube-shaped senor in it. Leave it for a few hours. 

On the following day, take the sensor out of the gas and dry it with a cloth. The senor may now be installed again. If the sensor doesn’t work, get a new one and try again.


Why does my idle go down when I brake?

An idle air control valve that has failed might explain this. During a fuel injection, the air intake is monitored by an idle air control valve. This is placed on or near the intake manifold.

Why does my RPM fluctuate when braking?

Be at ease if you notice an increase in your engine’s revolutions per minute (RPM) when you slow down or a brake. Braking slows down your automobile to a complete halt. Your RPM may arise when your car’s automatic gearbox tries to compensate for the shift in speed.

What causes a miss at idle?

Misfires and harsh idling may be caused by a variety of system problems. Lean mixes of air and fuel, as well as technical problems in the engine. These are all perpetrators of the ignition system’s malfunctions.

Bottom Line

Hope you got the answer to why your idle drops when break is pressed. You just need the appropriate tools and the correct processes to get the job done.

You might not confident in your ability to do the repairs. In this case, your vehicle serviced by a qualified technician. 

Wishing you the best of luck with your auto repairs!

Rob Dahm

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