# MK5 Brake Hissing



## imranjuma (Jun 14, 2017)

Hey Everyone, so I have a small problem with my 2007 Rabbit. I can hear a hissing noise coming from the cabin. The noise only appears when the brake pedal isn't pressed. When you press it in the noise goes away. Been looking on the forum and a lot of signs point to the brake booster. Can anyone confirm or have any insight on what might be happening. 


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## Fogcat (Apr 29, 2015)

Rubbing pads?


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## imranjuma (Jun 14, 2017)

Do you have the same sound in your car?


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## Thy_Harrowing (Dec 7, 2014)

I am fairly certain that the noise you are describing is the same noise I've had in my cabin ever since I took ownership of my car over 3 years ago. I wouldn't personally call it a overt "hissing" sound but it does sound very faintly of air doing something/going somewhere in the driver side front corner of the cabin and is only audible in the cabin. It goes away as soon as you press on the pedal enough. If you only lightly press on the pedal it gets even fainter but does not go completely away... correct? 

Despite the fact that I have never had any issues whatsoever with brake behavior I still have researched and physically investigated the source of the noise (I'm anal like that) and have come up with nothing definitive to explain the noise. It is definitely from the brake booster system but as to the "why" it is audible, I have nothing. All I can do is tell you that, in my experience, it is absolutely nothing to worry about. Again, over 3 years and counting and the noise has never changed in it's behavior nor have I had any reason to think it was indicative of an issue with the braking system. I've overhauled my entire brake system in that time with a big brake kit, better master cylinder, etc. and it is still there just as before. Definitely brake booster related but I'm positive, at least on my car, it is no issue at all so if what you're hearing is what I'm hearing then I'd say just put it out of mind.


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## imranjuma (Jun 14, 2017)

Thy_Harrowing said:


> I am fairly certain that the noise you are describing is the same noise I've had in my cabin ever since I took ownership of my car over 3 years ago. I wouldn't personally call it a overt "hissing" sound but it does sound very faintly of air doing something/going somewhere in the driver side front corner of the cabin and is only audible in the cabin. It goes away as soon as you press on the pedal enough. If you only lightly press on the pedal it gets even fainter but does not go completely away... correct?
> 
> Despite the fact that I have never had any issues whatsoever with brake behavior I still have researched and physically investigated the source of the noise (I'm anal like that) and have come up with nothing definitive to explain the noise. It is definitely from the brake booster system but as to the "why" it is audible, I have nothing. All I can do is tell you that, in my experience, it is absolutely nothing to worry about. Again, over 3 years and counting and the noise has never changed in it's behavior nor have I had any reason to think it was indicative of an issue with the braking system. I've overhauled my entire brake system in that time with a big brake kit, better master cylinder, etc. and it is still there just as before. Definitely brake booster related but I'm positive, at least on my car, it is no issue at all so if what you're hearing is what I'm hearing then I'd say just put it out of mind.


Wow thanks man this has my head spinning for a while. Thanks for the reply! 


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## Thy_Harrowing (Dec 7, 2014)

No problem man. Yeah it was a bit concerning to me for awhile too but after enough time and work on the brake system with no issues cropping up I figure it can't be a problem. I haven't been in enough VWs of the same year to say if they all do it but they might and it just goes unnoticed. It's easy to miss for the unattentive.

If you ever need anything else just PM me. Have a good one


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## PowerslavePA (Dec 9, 2014)

Does it only "Hssssss" when you push in the pedal, when pedal movement stops, then it stops? 
Or does it have a continuous "ssssssss" as you're riding the brake?

Older cars, the diaphragms hiss when you press the pedal in. If there is a leak,
vacuum leak, a small one, it will hiss continually. This happens with power brakes
when they get old. 

I had a car where the diaphragm went bad around the brake pedal piston, and 
made a continuous hisss when my foot was on the brake. When it was out,
there was no hiss, only when I ahd pressure on the brake. It started out as a
momentary Hsssss when I braked, then it for worse over time, till it finally
leaked. Did not effect braking till there was the continuous leak, but I could 
still manage... Eventually, I had to disconnect the vacuum hose to the brake
booster, plugged the engine side, and left the booster side open. Thus, I had
manual brakes...









This is an OLDER system, like from a 2000 jetta, yes, but this was what I had when it happened. 

If you see the big booster there, you see the pedal piston on the right, center, that
was where the hiss came from... You see the vacuum hose; that was what I
removed, plugged the hose from the motor, and left it open on the booster so
air would still move in and out with the brake,. If you plug that, you would not be
able to brake, or you'd blow out the diaphragm completely, still no brakes...

Here is an MK5 system:








Similar, just a smaller booster, but the setup is the same... Disconnect the vacuum hose
to the booster, and see if you still get the hisss sound, or if it changes. This pic only shows
the hydraulic clutch, but the booster and master are the same. I could not find a pic with
the brake pedal, but the diaphragm is on the interior side of the booster...


Here is a close-up of the booster:








This shows the interior side of the booster, plainly shows the boot, over the diaphragm..
You wanna check there for the hissing...


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## Thy_Harrowing (Dec 7, 2014)

@PowerSlave

Great info but thing he and I are talking about are basically the opposite of what you're getting at. 



> The noise only appears when the brake pedal isn't pressed. When you press it in the noise goes away


As far as I've worked out he and I have the same noise and I've already checked everything there is to check short of gutting the brake booster itself. My noise has been there steady, and unchanged for 3 years without a problem. If he had said the noise happened when he pressed the brake pedal I would have told him the same thing you did but the noise he and I hear goes away when pressing the pedal and does not emanate from any brake booster connection, rather it seems to just be coming vaguely from the driver's side corner, more at dash level not pedal level (oddly).

I think it's probably just a normal functioning noise from the brake booster that 95% of people don't even hear because only a small percentage of people are attentive enough to notice. It's a subtle noise.


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## imranjuma (Jun 14, 2017)

Thy_Harrowing said:


> @PowerSlave
> 
> Great info but thing he and I are talking about are basically the opposite of what you're getting at.
> 
> ...


I agree the noise is so small you could only hear it if you A) Knew about it B) Where looking for it 


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## PowerslavePA (Dec 9, 2014)

I just don't see why it would hiss when not pressing the brake, because it's a sealed
system when everything is working properly... Vacuum in the booster should not make
any noise unless there is a leak Still, whatever it may be... I just posted an issue I had...


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## radlynx (Jan 4, 2007)

i have had the same hissing sound a few years ago and fixed it by replacing the booster. 

the hissing sound is coming from the bad seal facing the driver or under the dash that pushes the rod when braking.

you may not notice about your braking being affected but that lost air or vacuum is needed by the engine for something else for proper engine operation and braking operation. get it fixed when you get the chance. 

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## petethepug (May 25, 2001)

Our 07 New Beetle had a hiss like a balloons last breath and then we got manual brakes.

It’s almost silent with no action, but applying the brakes gives a nice mini turbo blow off valve sound. Prolly our brake booster that died.


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## Thy_Harrowing (Dec 7, 2014)

radlynx said:


> i have had the same hissing sound a few years ago and fixed it by replacing the booster.
> 
> the hissing sound is coming from the bad seal facing the driver or under the dash that pushes the rod when braking.
> 
> ...


Yeah see that's the thing... I can't speak for the OP but my noise isn't coming from there. It is not coming specifically from where the rod on backside of the pedal pushes into the booster. Trust me, I checked, it is just a vague and subtle whirring behind dash that does not directly originate from that location. Nor do I have any loss of vacuum, nor has my braking changed whatsoever in 3 years. I'm convinced it is innocuous and can be largely ignored unless vacuum does clearly suffer and/or braking changes.




> It’s almost silent with no action, but applying the brakes gives a nice mini turbo blow off valve sound. Prolly our brake booster that died.


 Lot's of assisted/boosted brake systems these days make a little but of an air compression noise when you first push on the pedal. That is normal for many systems. BUT, and it's a big BUT, that is assuming that the noise you described as "a nice mini turbo blow off valve sound" is, in fact, the same noise I am describing. The fact is describing noises on a forum and getting any manner of consistency between what one person is thinking of and then tries to describe and what everyone else is thinking of when they read that description.... is very, very error prone at the end of the day. So what I have in mind as the "normal" noise above when first operating the brakes on assisted brake systems, may be different then what you're describing; you may be hearing something that is not normal and we'll never really know because we're trying to type about it haha.


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## radlynx (Jan 4, 2007)

Thy_Harrowing said:


> Yeah see that's the thing... I can't speak for the OP but my noise isn't coming from there. It is not coming specifically from where the rod on backside of the pedal pushes into the booster. Trust me, I checked, it is just a vague and subtle whirring behind dash that does not directly originate from that location. Nor do I have any loss of vacuum, nor has my braking changed whatsoever in 3 years. I'm convinced it is innocuous and can be largely ignored unless vacuum does clearly suffer and/or braking changes.
> 
> 
> Lot's of assisted/boosted brake systems these days make a little but of an air compression noise when you first push on the pedal. That is normal for many systems. BUT, and it's a big BUT, that is assuming that the noise you described as "a nice mini turbo blow off valve sound" is, in fact, the same noise I am describing. The fact is describing noises on a forum and getting any manner of consistency between what one person is thinking of and then tries to describe and what everyone else is thinking of when they read that description.... is very, very error prone at the end of the day. So what I have in mind as the "normal" noise above when first operating the brakes on assisted brake systems, may be different then what you're describing; you may be hearing something that is not normal and we'll never really know because we're trying to type about it haha.


im referring to constant hissing sound inside the cabin. its the booster causing it. also, when i press the brakes, i dont have any hissing sound. once again, a hiss sound when braking is a problem with the booster. my old booster started hissing at around 100000 miles. 

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## Thy_Harrowing (Dec 7, 2014)

That is my same issue as well, however, I would describe it as more of a whirring noise not a straight up hissing noise. It is not clearly coming from the brake boost or pedal area, it is pretty much vague in terms of where it is coming from. It does indeed go away when pressing the pedal. It has been occurring since the very beginning and I'm about to hit 90K, and it has not change one damn bit in all that time, nor has my braking changed (except when I upgraded to StopTech ST40, naturally).

I can speak for y'all's situation but in my case, it is innocuous. I am not concerned.


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