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I drive a Ford Fiesta Classic, 2008 model, clocked over 97k kms.

Yesterday, When I left office at night, the surrounding was very silent and I observed a hissing sound coming from near the pedals below steering. I suddenly pressed the brake and stopped the car, but the sound disappeared. When I released the brake pedal, the sound again came. This sound is related to the brake pedal for sure.

The sound is still on even with hand brake engaged. Only a pressed brake pedal stops this sound. I don't find any decrease in braking performance, till now.

I showed my car to a local garage today morning, he said it is okay. But, as it is related to braking system, I am worried.

Anyone had a similar issue or know about this, please give me suggestions.

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That sounds to me like a vacuum leak. Most cars have servo-assisted brakes (the reason it's much harder to brake if the engine is switched off), which is usually powered by a vacuum line taken off the inlet manifold (so that as the engine sucks in air and fuel, it also sucks air out of the servo).

If you look in the engine bay at the point nearest the brake pedal, you'll see the servo - a large round thing about the size and shape of a small casserole, with the brake cylinder stuck to the middle of it. There should be a hose going into this from somewhere. Check all round there for signs of the leak...

  • Spot on! brother – Shobin P Jul 16 '15 at 8:48
  • With the engine running, I am unable to hear the sound from engine bay. Anyways, I will look for that hose which u mentioned. – Rohith Jul 16 '15 at 9:19
  • @Nick, Is it okay to go on with it as the guy from the garage suggests? – Rohith Jul 16 '15 at 10:23
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    It should be - the likely side effect of the vacuum failing altogether would be a loss of servo assistance, so the brakes would go very heavy, but still work. However, if you're worried, take it to another garage for a second opinion (Don't rely on strangers on the internet for safety critical advice ;) ) – Nick C Jul 16 '15 at 10:26
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Following Nick's answer...

There is sometimes a vacuum line that goes inside the cabin of the vehicle for HVAC controls. This line usually comes from the brake booster. Look at the brake booster's vacuum lines and see if there is one going through the firewall. If there is, try to follow it through the firewall and under the dash to the HVAC controls to see if you can find a leak. It might be easier to cap off the line at the brake booster.

A couple tools that can be useful in finding vacuum leaks: if a hose has a leak and you spray it with carburetor cleaner, the engine should rev up slightly.

A stethoscope will help you pinpoint where a noise is coming from

a vacuum gauge can help you determine if a line in leaking. If capping off a line causes higher vacuum, then there is a leak.

A handheld vacuum pump will allow you to test lines/devices. Connect the pump and pull about 20" of vacuum. If the vacuum drops, there is a leak.

  • I remember the guy from local garage mentioning brake booster. If I press the brake pedal while starting the engine, it revs up slightly above normal. Does this action denotes anything? – Rohith Jul 16 '15 at 15:05
  • A vacuum leak being introduced will cause the engine to rev up. If your in your car, stopped, in park (or neutral with the emergency brake on), and hit the brake pedal, does the engine rev up? BTW - the brake booster is the servo assist device Nick was referring to. – rpmerf Jul 16 '15 at 15:17
  • No, the engine does not rev up on brake pedal press. I still didn't find the source of this sound. Anyways, I am taking my car to another garage tomorrow. – Rohith Jul 17 '15 at 7:25
  • Please get back to us and tell what was done to rectify the situation. – Sam Rosario Jan 3 '16 at 7:29
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If it is a constant hissing sound I would be inclined to think along the lines of a vacuum issue. I suggest you do a Power Brake Booster test to determine if the Booster is beginning to fail. You will know this if you turn on the car, press the brake down and hold it for about 15 seconds to see if the pedal floors out (if it floors out you may need a new Master Cylinder. if you hear a "Whaught" sound (a vacuum sound in reverse) coming from under your dashboard when releasing the brake pedal then chances are you may need a Power Brake Booster.

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Whenever your brake booster makes a hissing sound, don't waste time or money trying to fix it, just replace it. I had the same problem with my 2001 Tacoma. It sounded like I had a wild horse under the dash when I pressed the brake pedal. I took out the booster and pulled it apart. There was a dried out seal at the very front that was making the sound.

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Notice how no one here actually diagnosed vacuum leaks in the brake booster. If remove and plug the vacuum line to the brake booster this removes all vacuum to the booster. Then, run the engine and listen for the "snake hissing" vacuum leak or in other cases the thud sound of the booster engaging. If these sounds go away but return when plugging the vacuum booster back in, replace the booster. If there is paint wrinkling or oil leaking from the bottom of the brake booster but the booster checks out as OK then replace the mastercylinder.I HIGHLY recommend buying a booster attached to a master cylinder as a unit. Most boosters are fed engine vacuum from the engine side of the fire wall. Also, you can use any simple vacuum gauge to track down where your leak is coming from. First check normal vacuum when the engine is running so you have an understanding of what normal should be. Then start back tracking from the engine. Sometimes you can connect T-ports inline to adapt in your vacuum gauge. Then pinch off the vacuum supply between the gauge and the engine. If you lose vac vacuum your leak is in the circuit with the gauge. Alternatives are to use carb cleaner, propane/ butane, or lysol spray (contains alcohol). All these are highly flammable so take my disclaimer that you need to know what you are doing and be very careful and familiar with these techniques & their ability to cause fire or get a professional.

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