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Topic: Air "hissing" noise when applying brakes. (Read 21550 times) previous topic - next topic

Air "hissing" noise when applying brakes.

I left work really late last night, and while driving home as one of the only couple of cars on the road I noticed when I apply my brakes, I can hear a very faint hissing noise under or in my dash someplace. I moved my hand around down there and didn't feel any air. The engine doesn't stumble or cut out of anything, but the hissing does increase the harder I step on the brake.

I never noticed it before, but I was curious if this is normal. It may be, the brakes work fine, I don't notice anything different about them. Do you guys have this too? What should I check?

There was a thread a year or more ago about a similar problem, I remember the guy had a column shift car and he could hear a wind leak or a hissing or something from under his dash, but I can't find the thread. I think he figured it out. Can't find it, maybe it was deleted.

Air "hissing" noise when applying brakes.

Reply #1 –
Theres a vacuum hose that attaches to the brake pedal-it's for the cruise control. Check if that came off or is broken. Or maybe your brake booster is leaking.

Air "hissing" noise when applying brakes.

Reply #3 –
Quote from: jpc647;350823
There was a thread a year or more ago about a similar problem, I remember the guy had a column shift car and he could hear a wind leak or a hissing or something from under his dash, but I can't find the thread. I think he figured it out. Can't find it, maybe it was deleted.

I'm pretty sure I posted in that thread, I had the same problem. I don't remember it ever getting resolved. However, I also don't think it has anything to do with what you're talking about, because the problem I was experiencing was a specific draft that could not be sourced, directly related to vehicle speed and nothing else.
--SteveN 👍
[thread=28690]1988 Cougar V6[/thread]
2012 F-150 3.7L

Air "hissing" noise when applying brakes.

Reply #4 –
It sounds like the vacuum bleeding off from the cruise control. I think it's normal to a degree. The vacuum bleeds off for safety in case the brake doesn't release the cruise, like if the brake light switch failed (not that that would ever happen, right?)
2017 Subaru Outback
2007 Lincoln MKZ
1990 Ford F250 / Banks turbo diesel

Air "hissing" noise when applying brakes.

Reply #5 –
I hate to be the bearer of bad news, but hearing a hissing sound when you step on the brakes is indicative of a leaking diaphragm in your brake booster.  Without being there to diagnose your car, I can almost guarantee you have a bad booster.
William

Air "hissing" noise when applying brakes.

Reply #6 –
By the way; I don't know whether you knew this, or not, but whenever you replace a brake booster, always leave the check valve that comes with the booster, in the booster.  If you remove the new check valve and reuse your old check valve that was sticking into your old booster, your new booster's life in most cases will be dramatically shortened.
William

Air "hissing" noise when applying brakes.

Reply #7 –
Two things,

brake booster


or,,, and listen close


AUTO BRAKE RELEASE!!!

yep, my 20th is equipped with auto emergency brake release.  When it is engaged, and you shift from park to drive, the Ebrake auto releases.
There is a check valve looped back above the brake pedal that will leak if its faulty.
Mine starting hissing more than my ever so familiar brake booster hiss so I looked into it and found my leak.  I no longer have auto Ebrake but do have the parts.

As for the brake booster,,,, i think its nothing more than welcome to the world of fords,, a little hiss is pretty much the norm on more than 50% of us here.

The cruise advice you got is not correct, all cruise vac hoses are isolated to the engine bay and out to the driver fender.  The only cruise applications are electrical on the interior of the car.

Air "hissing" noise when applying brakes.

Reply #8 –
Now that I think about it, the vacuum lines are too small for the vacuum break to make any significant noise. I wasn't aware that it was routed out of the interior. I can't say I've heard it on my Cougar. One of my other cars (I think it was my Diamond Jubilee T-bird) had a bit of a hiss, but I never paid it much mind. Always thought it was the crusie control. Learn something new every day.....
2017 Subaru Outback
2007 Lincoln MKZ
1990 Ford F250 / Banks turbo diesel

Air "hissing" noise when applying brakes.

Reply #9 –
Quote from: jcassity;350864
Two things,

brake booster


or,,, and listen close


AUTO BRAKE RELEASE!!!

yep, my 20th is equipped with auto emergency brake release.  When it is engaged, and you shift from park to drive, the Ebrake auto releases.
There is a check valve looped back above the brake pedal that will leak if its faulty.
Mine starting hissing more than my ever so familiar brake booster hiss so I looked into it and found my leak.  I no longer have auto Ebrake but do have the parts.

As for the brake booster,,,, i think its nothing more than welcome to the world of fords,, a little hiss is pretty much the norm on more than 50% of us here.

The cruise advice you got is not correct, all cruise vac hoses are isolated to the engine bay and out to the driver fender.  The only cruise applications are electrical on the interior of the car.

If the brake boosted was leaking, wouldn't the car have a constant stumble or rough idle, especially when applying the brakes? -- I'm just curious. My car doesn't have the auto Ebrake thing, hell I don't even have an ebrake anymore. All the lines are there and it appears to work, but it doesn't.  But you too get a little hiss in your 20th? How loud would you say it is? Just for comparison. Now that I know it's there, I can hear it during the day, but with traffic and all the city noise it is very faint.

Air "hissing" noise when applying brakes.

Reply #10 –
Quote from: BadTriumph;350872
Now that I think about it, the vacuum lines are too small for the vacuum break to make any significant noise. I wasn't aware that it was routed out of the interior. I can't say I've heard it on my Cougar. One of my other cars (I think it was my Diamond Jubilee T-bird) had a bit of a hiss, but I never paid it much mind. Always thought it was the crusie control. Learn something new every day.....

This isn't a significant noise. You really have to listen for it, driving to work during the commute, it's barely loud enough to hear.

Air "hissing" noise when applying brakes.

Reply #11 –
Quote from: jcassity;350864
The cruise advice you got is not correct, all cruise vac hoses are isolated to the engine bay and out to the driver fender.  The only cruise applications are electrical on the interior of the car.

Not true.
The vacuum dump valve is controlled by the brake pedal. It is a "normally open" valve that is held closed by a pad on the brake pedal. When the pedal is pushed the pad moves away from the valve, allowing it to open.
The dump valve vacuum line may look like it is going to the vacuum tree, but it goes through a hole in the fire wall just below the tree.

Air "hissing" noise when applying brakes.

Reply #12 –
^ Interesting... I'll look for it and try to see if that is where it is coming from.
 

Air "hissing" noise when applying brakes.

Reply #13 –
softtouch, thank you for setting it right.
sorry bout that, it totally slipped my mind,, not sure why either cause its so obvious. 

JPC
if its a minor hiss then its your brake booster like my LS.  you can hear it when you put the car in park and manually move the pedal with your hand and zooom in on the noice area.

the Vac leak i had on the auto e brake was fairly loud and was causing high idle and causing the car to lurch forward when rpms were gained by the vac leak.

again, sorry about my mistake.