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Hissing noise when brake pedal is depressed Inspection Service

Brakes are the most important safety components on your vehicle. Despite that importance, it’s easy to take them for granted, at least until your car begins to do strange things. Ordinarily, pressing your brake pedal should result in only the car slowing down or stopping. If you hear a hissing noise when the brake pedal is depressed, it may mean there’s something that needs to be repaired in the system.

How this system works:

On many cars, the master cylinder works in tandem with a brake booster. The brake booster is mounted to the firewall, generally in front of the steering wheel position, and the master cylinder attaches to it. The entire point of the brake booster is to make it easier to press the brake pedal and slow or stop the car.

Your brake booster works on vacuum pressure. There’s a diaphragm inside that is responsible for maintaining pressure when you’re not pressing the pedal. However, when you do press the pedal, a push rod moves forward within the booster and into the master cylinder. The side of the diaphragm facing the cabin vents to atmospheric pressure, while vacuum is maintained on the other side.

This then equalizes with pressure on the other side of the diaphragm, providing the boost needed to make pressing your brake pedal easier. The hissing noise you hear when the brake pedal is depressed could be nothing more than ordinary operation, or it could be a sign of a deeper problem. There are a couple of different potential issues here, some more severe than others.

Common reasons for this to happen:

  • Leaking Vacuum Line: Your brake booster is supplied with vacuum from the engine via a vacuum hose. It’s usually located near the firewall. If this hose has a leak, it can be heard within the cabin.

  • Leaking Brake Booster Diaphragm: The diaphragm that’s responsible for maintaining pressure within the brake booster can fail, particularly if the master cylinder is leaking fluid into the booster. When this happens, you’ll hear a hissing sound when depressing the brake pedal, but the pedal itself will be much, much harder.

  • Damaged or Missing Foam Silence: Most cars equipped with brake boosters also have a foam silencer that’s designed to help prevent you from hearing that hissing sound. If the silencer has degraded or been damaged, you’ll hear the hissing sound.

  • Failing Master Cylinder: If you hear a hissing sound and suspect that the problem is the diaphragm in the booster, the actual cause is likely the master cylinder beginning to fail. Signs that the master cylinder is leaking into the brake booster include low fluid in the reservoir with no visible leaks.

What to expect:

A top-rated mobile mechanic will come to your home or office to inspect your brake system, including the brake pedal operation, the brake booster and listen to the hissing sound. The mechanic will then provide a detailed inspection report that includes the scope and cost of the necessary repairs.

How it's done:

The mechanic will need to crank the car in order to verify the hissing sound, and a test drive may be necessary to duplicate the complaint. Additionally, the mechanic will inspect the brake pedal operation, the condition of the brake booster, the master cylinder, the level of fluid in your system and more.

How important is this service?

Power brakes are wonderful things, making it far easier to work a brake pedal that would otherwise require significant force to activate. While the hissing sound may be a sign of nothing more dangerous than a failed piece of foam, it could be a symptom of something much more worrisome – a damaged brake booster and a failing master cylinder. You cannot afford to take chances with your brake system. One of our professional mechanics can inspect the entire system and determine if there’s cause for concern, and what needs to be done if there is a problem.


Recent Hissing noise when brake pedal is depressed Inspection reviews

Excellent Rating

(13)

Rating Summary
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Peter

25 years of experience
514 reviews
Peter
25 years of experience
Jeep Patriot L4-2.0L - Hissing noise when brake pedal is depressed Inspection - Sylmar, California
Peter was amazing and really informative on what my car needed. He arrived on time and was quick to get the work done. I definitely recommend him to anyone needing a great mechanic!

Jose

9 years of experience
63 reviews
Jose
9 years of experience
Volkswagen GTI L4-2.0L Turbo - Hissing noise when brake pedal is depressed - San Francisco, California
Jose communicated, was on time, and did excellent work. I would happily have Jose work on my car again in the future.

Matthew

14 years of experience
15 reviews
Matthew
14 years of experience
Lincoln MKX V6-3.5L - Hissing noise when brake pedal is depressed Inspection - Cleveland, Ohio
Matthew called and asked if he could come early because he was ahead of schedule. Matthew was very courteous and gave me an accurate diagnosis. I appreciate Matthew not inflating the problem and not fabricating a problem that was unnecessary. Great , convenient experience.

Brian

21 years of experience
109 reviews
Brian
21 years of experience
Chevrolet K1500 V8-5.7L - Hissing noise when brake pedal is depressed Inspection - Glendale, Arizona
Awesome mechanic. Went to bat for me regarding length of time repair was estimated to take vs. actual time. Saved me over $100. Highly recommend

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