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Common Symptoms of a Faulty Brake Booster

By R&D
Published on May 5th, 2025

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The Brake Booster is a critical component of your vehicle’s braking system. The Brake Booster is located between the brake pedal and the master cylinder. Its primary purpose is to amplify the force applied to the brake pedal, allowing the driver to engage the brakes with less effort. A Brake Booster typically consists of a diaphragm, coil springs, pushrod, valve body, and a vacuum chamber (in vacuum-assisted systems).

Common Symptoms of a Faulty Brake Booster

When the brake pedal is pressed, the valve body opens to allow atmospheric pressure into the vacuum chamber. This pressure difference pushes the diaphragm forward, amplifying the force transmitted to the master cylinder. This amplified force increases hydraulic pressure in the brake lines, enabling the calipers or drum brakes to efficiently slow or stop the vehicle. A faulty Brake Booster can compromise braking performance, leading to increased pedal effort, reduced braking power, and potential safety risks. Neglecting timely replacement of a faulty Brake Booster can compromise safety and increase repair costs.

This article highlights the typical symptoms of a faulty Brake Booster, to help you identify potential issues and their root causes.

What Causes Brake Booster Failures?

Vacuum Leaks: Cracked or disconnected vacuum hoses, damaged check valves, or leaks in the vacuum chamber can compromise the vacuum pressure needed for the Brake Booster to function effectively. This is a common issue in vacuum-assisted Brake Boosters and can lead to inconsistent braking performance. Over time, even small leaks can cause a significant drop in vacuum pressure, making the brake pedal harder to press.

Diaphragm Damage: The diaphragm inside the Brake Booster can wear out, tear, or lose elasticity over time. This prevents it from creating the pressure differential needed for force amplification. Such damage is often caused by prolonged use, exposure to harsh conditions, or contamination, and it typically results in a spongy or unresponsive brake pedal feel.

Faulty Check Valve: The one-way check valve, which prevents the backflow of air into the vacuum chamber, can fail due to clogging or wear. A faulty valve disrupts the Brake Booster’s ability to maintain vacuum pressure, especially during heavy braking or engine shutdown. This can also cause a delay in braking response, as the Brake Booster cannot build or maintain sufficient vacuum quickly enough.

Wear and Tear: As with any component, the Brake Booster is subject to wear over time. Prolonged use can degrade internal parts such as seals, springs, and rods, leading to diminished performance. Older vehicles are particularly susceptible to this issue, especially if the Brake Booster has never been serviced or replaced.

Contamination or Corrosion: Moisture or dirt can enter the Brake Booster through damaged seals or air filter, leading to internal corrosion or clogging of components. Over time, this can damage critical parts such as the diaphragm or springs, reducing the Brake Booster’s effectiveness and potentially leading to complete failure.

Rough Driving Behavior: Aggressive driving habits, such as frequent harsh braking or sudden stops, place excessive stress on the braking system, including the Brake Booster. Over time, these behaviors can result in premature wear or failure of the Brake Booster’s internal components, reducing its efficiency and reliability.

Common Symptoms of the Faulty Brake Booster

Malfunction Indicator Light (MIL) or Check Engine Light (CEL) Illuminates

A malfunctioning Brake Booster may trigger the vehicle's On-Board Diagnostic (OBD) system to illuminate the MIL and store a corresponding Diagnostic Trouble Code (DTC) in its memory. However, the MIL can be illuminated for multiple reasons, so using a Scan Tool to diagnose the specific issue is essential.

Hard Brake Pedal

One of the earliest and most noticeable signs of a faulty Brake Booster is the increased effort required to press the brake pedal. Without the Brake Booster’s assistance, the driver must exert significantly more force to engage the brakes, leading to a hard or unresponsive pedal feel.

Whistling or Hissing Noises While Braking

A damaged diaphragm or vacuum leak within the Brake Booster can cause whistling or hissing sounds, especially when pressing or releasing the brake pedal. These noises are often due to air escaping from the vacuum chamber and clearly indicate an internal or external vacuum-related issue.

Engine Stalling During Braking

A vacuum leak in the Brake Booster can disrupt the air-fuel mixture by allowing unregulated air into the intake manifold, leading to rough idling, reduced engine performance, or stalling, especially when braking at low speeds or while stationary. In cases of a large vacuum leak, the Brake Booster may draw excessive vacuum when the brakes are applied, depriving other components of the necessary vacuum. This imbalance can cause the engine to stumble, result in a drop in RPMs, and potentially lead to a complete stall.

Poor Braking Performance

A failing Brake Booster reduces the hydraulic pressure transmitted to the braking system, diminishing overall braking power. This often results in poor braking performance, thus leading to extended stopping distances, even during normal driving conditions, creating a potential safety hazard.

Find the perfect scanner in 1 minute

Identify a Faulty Brake Booster with the Innova Scan Tool

The Innova Scan Tool quickly retrieves and identifies existing DTC(s) stored in your vehicle’s computers. With a quick OBD2 Scan, you can retrieve existing DTCs, the I/M Status of the vehicle, and any collected Freeze Frame data from these computers. Refer to the Innova Coverage Checker to determine the level of information supported on your vehicle by year, make and model. The Innova Scan tool effectively diagnoses Brake Booster-related issues, helping you easily pinpoint and resolve problems.

Additionally, the Innova Scan Tool lets you view Live Data PIDs and presents OEM Enhanced Live Data, with advanced information for complete vehicular diagnostics. You can also read the Brake Booster Vacuum Pressure value with the Innova Scan Tool to determine if the Vacuum Pressure value matches with the actual vacuum pressure or if it changes after many movements. The normal operating value of the Brake Booster Pressure Sensor depends on your vehicle.