# Hissing brake pedal



## devildarky2003 (May 19, 2008)

Hello

I pulled into work this morning and turned the radio off and noticed when i press my brake pedal i get a hissing noise also when the pedal comes back up.

When the engine is turned off i pressed the pedal a few times and it went hard so i couldnt press it.

Any ideas what this could be? brakes seem to work as normal.

Thanks


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## Hoggy (May 8, 2002)

Hi, Could be a air leaking into brake servo vacuum pipe, caused by a split in pipe..
Brake pedal will be hard with engine off as there is no vacuum to operate brake servo.
Hoggy.


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## devildarky2003 (May 19, 2008)

Thanks

Is this a problem that needs sorting?


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## Hoggy (May 8, 2002)

Hi, Yes needs sorting if it is a split in vacuum pipe, as not only will it affect your brakes but will cause a weak mixture as extra air is being sucked into inlet manifold.
Hoggy.


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## nordic (Apr 26, 2010)

One strange thing here is that brakes could not be pressed after engine stopped, usually servo allows pedal to be pressed few times before it gets stiff. It well could be a servo gone, say, membrane is blowing...

Kind regards


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## Hoggy (May 8, 2002)

nordic said:


> One strange thing here is that brakes could not be pressed after engine stopped, usually servo allows pedal to be pressed few times before it gets stiff. It well could be a servo gone, say, membrane is blowing...
> 
> Kind regards


Hi,If the servo pipe is split then all vacuum will be immediately lost as soon as engine off.= hard pedal.
Hoggy.


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## devildarky2003 (May 19, 2008)

Any idea if its a lot of work and costs?

I have dean from 4 rings coming to look at my suspension next week maybe worth asking him to have a look at that as well.

But money is tight around xmas :?


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## nordic (Apr 26, 2010)

Right, thanks! You're right, sorry, I suppose there must be some kind of a one way valve then as well...


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## John-H (Jul 13, 2005)

For the vacuum servo to work it must allow air pressure in on the non vacuum side of the servo. That's the pedal side and usually there is some sort of filter to stop debris being sucked in as the diaphram moves forward with atmospheric assistance under control of the pedal. The filter is often just a simple piece of sponge around the pedal pushrod which takes in clean cabin air. If it's missing or worn you will hear more of the sound of the atmospheric valve opening allowing the air in.

When you release the pedal the air intake is closed and the atmospheric air pressure on the pedal side is allowed through into the vacuum part of the chamber - in other words an equal vacuum is created on the pedal side - so, with equal vacuum both sides of the diaphram there is no assistance. Without the engine running this action can only occur a few times before the vacuum has gone and no further vacuum assistance is possible. The fact that the pedal therefore stiffens up is normal.

If you have a leak on the vacuum connection to the servo it's less likely to be heard as it's under the bonnet but if so, you would likely loose all or part vacuum assistance and have engine running problems. If there was a split in the diaphram you would have no assistance but you may hear a continuous hiss when the pedal is pressed with the engine running.


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## devildarky2003 (May 19, 2008)

John-H said:


> For the vacuum servo to work it must allow air pressure in on the non vacuum side of the servo. That's the pedal side and usually there is some sort of filter to stop debris being sucked in as the diaphram moves forward with atmospheric assistance under control of the pedal. The filter is often just a simple piece of sponge around the pedal pushrod which takes in clean cabin air. If it's missing or worn you will hear more of the sound of the atmospheric valve opening allowing the air in.
> 
> When you release the pedal the air intake is closed and the atmospheric air pressure on the pedal side is allowed through into the vacuum part of the chamber - in other words an equal vacuum is created on the pedal side - so, with equal vacuum both sides of the diaphram there is no assistance. Without the engine running this action can only occur a few times before the vacuum has gone and no further vacuum assistance is possible. The fact that the pedal therefore stiffens up is normal.
> 
> If you have a leak on the vacuum connection to the servo it's less likely to be heard as it's under the bonnet but if so, you would likely loose all or part vacuum assistance and have engine running problems. If there was a split in the diaphram you would have no assistance but you may hear a continuous hiss when the pedal is pressed with the engine running.


Umm is that a big job then? :?

I read on other forums to try and turn the engine off press the pedal a few times till its stiff then hold my foot on the pedal turn the engine on and see if the pedal stays stiff or slowly goes down

My pedal goes down?


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## devildarky2003 (May 19, 2008)

Been reading online

Some people say its normal others saying its a big problem

But as above because the pedal goes down when i seitch the engine on tells me the booster is working fine?

So a split in the pipe sounds like that could be the problem? Anyway for me to check?


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## John-H (Jul 13, 2005)

devildarky2003 said:


> John-H said:
> 
> 
> > For the vacuum servo to work it must allow air pressure in on the non vacuum side of the servo. That's the pedal side and usually there is some sort of filter to stop debris being sucked in as the diaphram moves forward with atmospheric assistance under control of the pedal. The filter is often just a simple piece of sponge around the pedal pushrod which takes in clean cabin air. If it's missing or worn you will hear more of the sound of the atmospheric valve opening allowing the air in.
> ...


The... press pedal loads to get rid of vacuum, hold it pressed, start engine and notice the pedal descend slowly... is a test to see if vacuum is getting through and the servo is functioning to give you assistance. The... turn off engine, wait a while then press the pedal repeatedly to notice it stiffening up after a few presses ... is a test of the servo's ability to retain vacuum. It sounds like you pass both.

I presume the hissing noise only occurs as the pedal is moving and once it is held in position there is no hissing. A continuous hissing would indicate a leak through the diaphram but a momentary hiss as the pedal moves is just normal operation, the only issue being that you can hear it, which I suspect may be the filter missing or worn. You need to have a look at where the pushrod goes into the back of the servo. I'm afraid I've got no specific TT information about fitting a replacement filter - Etka doesn't mention it and it seems to be integral with the servo.

One possibility may also be that the pushrod is bent or in some way out of alignment, so it pulls the hole in the sponge into an oval so creating a gap down the side through which the hiss can be heard. It could also be some sound proofing has fallen away, or even that you've only just noticed it and are concerned. If you can first confirm the hiss only being associated with pedal movement and thenthat it's coming from inside and not in the engine bay then take the lower dash off - you will be able to have a poke around.

If the hiss happens all the time when the pedal is pressed there's a leak or if you loose vacuum assistance at all when driving there's likely a leak too - in which case this may help: viewtopic.php?f=2&t=93733&p=1009394&hilit=vacuum+diagram#p1009394


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## devildarky2003 (May 19, 2008)

John-H said:


> devildarky2003 said:
> 
> 
> > John-H said:
> ...


John Sir you are the man!!

Yes i do pass both test and can hear it in the cabin. Once held the hiss goes away only when pressed and re raising do i hear it. Hopefully not a major problem then (relief)

Ill have a look on saturday but not to sure what i am looking for :?


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## Chemlock (Mar 2, 2019)

I notice that there was no response on this thread for awhile. I too had this problem. Very slight hissing noise from the pedal area. Goes with the lightest press of the brake pedal. Returns as soon as you take your foot off the brake. I didn't spot it at first due to the exhaust sound on the TTS.

Very difficult to hear the issue when test driving a car so look out for it when buying second hand.

My car is now with Audi getting the brake server replaced as they are 99% sure this is the issue. At my local Audi dealer the job is £760 (but at least they wash the car for that). Hope this helps anyone else who finds this issue


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## Hoggy (May 8, 2002)

Chemlock said:


> At my local Audi dealer the job is £760 (but at least they wash the car for that).


Hi, I'd definitely want a discount if they are going to wash the car, to help pay for the machine polish afterwards.  
Hoggy.


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## SinfulDesignCom (Mar 2, 2013)

My pedal has been hissing for about 5 years!!

Passed all MOT's and never had a problem with mixture or emergency stoppage.

Split pipe was found last time Wak had a look. Replaced this. Hiss still there. No difference.

I have forgotten about it.


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## Outnumbered (Mar 8, 2015)

Hoggy said:


> Hi, Yes needs sorting if it is a split in vacuum pipe, as not only will it affect your brakes but will cause a weak mixture as extra air is being sucked into inlet manifold.
> Hoggy.


Hi Hoggy (or anyone)
I have a similar issue but the opposite to the OP ie I have a constant hissing which stops when I press the brake peddle. Also on the odd occasion I have a long/soft peddle.
What I'm struggling to understand is the connection with a weak mixture/extra air being sucked into the inlet manifold that you mention.
The reason this is of interest to me is because I feel I am lacking overall power low down in the rev range.
Thanks
Top


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## Outnumbered (Mar 8, 2015)

Hi Hoggy (or anyone)
I have a similar issue but the opposite to the OP ie I have a constant hissing which stops when I press the brake peddle. Also on the odd occasion I have a long/soft peddle.
What I'm struggling to understand is the connection with a weak mixture/extra air being sucked into the inlet manifold that you mention.
The reason this is of interest to me is because I feel I am lacking overall power low down in the rev range.
Thanks
Top


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## Hoggy (May 8, 2002)

Hi, Vacuum which is used by the brake servo is created in the inlet manifold so if there is leak in the hoses to the brake servo this unmetered air sucked into the inlet manifold causes the weak mixture.
Hoggy.


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## monty168505 (Sep 3, 2019)

I have the hissing sound all the times and goes off if I slightly press the brake pedal. Is this the same issue and can it be an easy fix? it's on the 180 audi tt mk1.

Sent from my SM-N960F using Tapatalk


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