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brake air

VladDracule

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Hello all,

I took my car into ford because the brakes randomly started "hissing" when i press them, parked or moving, car on or off their is a hissing noise of air near the brake peddle. Ford is telling my its normal which i dont believe at all. Ive owned multiple mustangs and none have ever experienced this, any feedback?
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JetGray_Mach1

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I experienced this on my 2014 GT and on my Mach 1. I think its normal as well, its part of the brake vacuum system. If you had a leak your brake pedal would go to the floor and your breaking would be limited. Also you would be loosing fluid.

Few things to check
1) with the car off if you press brake pedal it should go down soft and get hard, it should stay hard. If it doesn't stay hard it indicates a leak.
2) check brake fluid level and for any leaks
3) when the car is on and you depress brake pedal it should have assist then gets firm at the end. If it continues to go down to the bottom that indicates a leak.

BTW how many miles do you have on your car? You are saying this just started happening? try the steps above may point you to a leak if its present.
 

JetGray_Mach1

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spedy7

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You'll normally hear a bit of noise from the brake booster as up until the S650 it's been vacuum-type assist. The booster will draw air into the firewall-side chamber from the cabin and exhale through the cabin as the pedal is actuated to offset the engine-side chamber that is under (mostly) constant vacuum - this in most basic terms gives you the boost assist. Quickly depressing the brake will definitely give you that "hissing" noise. Pretty much all cars with a vacuum-assist type brake booster will exhibit it, some may be louder than others.

If it's constant (all the time), then there's a vacuum leak (or some other issue). Like said, you can also check if the booster is naturally leaking down by checking the brake pedal feel the next day with the car off - the vacuum should hold overnight (check with engine off). Extended pedal travel due to leaks may cause it to be more noticeable, but I don't have that much experience with brake systems to confirm.

You could try replacing the booster, but in my opinion if the brake pedal feel is ok and the noise isn't super loud I wouldn't replace anything.
 

JetGray_Mach1

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I posted the video if it sounds like mine it’s normal
 

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Ole Texan

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If that is normal it is a pizz poor design. I have owned a lot of cars with boosted brakes and never had one hiss at all. If you know a good local shop have them listen to the hiss and give you feedback. I don’t trust any stealer dealers. There first answer is almost allways “that is just normal” unless there is a rod hanging out the block.
 

Paul McWhiskey

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Mine sure does not sound like that. I get no noise at all when brake is applied.
 

Cobra Jet

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I've never heard that type of "brake air" or "hissing" in any S550 when applying the brakes.

Take it to another Ford Dealer or you can call the Ford 800# and open a case with a Regional Rep. to state your dissatisfaction with the repair efforts.
 

melwff

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Older cars used a foam noise filter where the brake pedal push rod goes through the firewall. If Ford still uses them, it is either missing or not correctly installed. Try the parts department to see if the filter is still used.
 

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paulm1

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Check your manifold vacuum when you step on the brake's at idle, it might fluctuate an inch or so then be right back to where it started. If it drops and stays down there's a leak in the booster.
 

NightmareMoon

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Some hissing noise is normal on the S550. A lot of hissing noise isn't normal.

I had a situation last year on my 90k mile hard-use 2016 where I was getting a lot of noise and a lot of soft pedal travel and it was a recent development. Not confidence inspiring at all.

Replaced my master cylinder at some expense and it went back to a normal amount of noise. It wasn't night and day different after the repairs, but it improve noticeably. There's a lot less sound, but there is still some.
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