Audi a6 2005 auto v6 brake problem

[Stephend161]

Hi fellow Audi fans..

so i have a friend who has a 2005 Audi a6 s line v6 auto..

today on a drive around 25/35 mins in he experienced brake loss..

he described a loud wining noise followed by real bad brake loss..

the brake pedal he says still had pressure but the brakes was very weak.

he then said the brake pedal dropped and he had to pump the pedal a few times to build up any brake pressure to then be able to stop the car…

he managed to get the car back home and then later on went out in the car and said the brakes was back to normal however this intermittent problem has now occurred twice in the last few months.. the car does not get used much but seems to now be doing this intermittent problem every time the car is taken out after 25/35 mins of driving.

today i checked and there is no lights on  the dash and no codes stored in the cars abs or ecu module atall as i scanned the car with a diagnostics computer and nothing comes up..

When this first happened a few months ago another friend checked all 4 corners and said there is no problem with any callipers or disks or pads or abs sensors  and he said he found no obvious problems nothing looked to be obviously wrong.

he did give the car a brake fluid change to which he thought it had solved the problem as the brakes seemed ok after however we now know this to just be an intermittent problem as the same issue has shown itself today...

the brake fluid is about 2mm from max so its not low.

Could anyone shed any light on what this could possibly be or give me some ideas as to what i can check to try and sort this problem..

thanks all 

 

[Magnet]

Hello Stephen,

Many thanks for joining and posting your friend’s plea. 
Worth checking the brake servo operation - and easy self-test which you can Google. 
Of course, it is likely to test OK when it’s OK! - but the test would need to be carried out again when the issues arises. 
Kind regards,

Gareth. 

[Stephend161]

1 hour ago, Magnet said:

Hello Stephen,

Many thanks for joining and posting your friend’s plea. 
Worth checking the brake servo operation - and easy self-test which you can Google. 
Of course, it is likely to test OK when it’s OK! - but the test would need to be carried out again when the issues arises. 
Kind regards,

Gareth. 

Hi thanks for the reply..

we will do a test next time i go up and will post findings.

would you have any idea what could be causing the high pitched noise he is hearing that arises when the fault happens.

[Magnet]

Hello Stephen, 

Could be backing up wrong trees and older technology-? - servos can be operated by vacuum, so noise originates from loss of vacuum, and if so reason has to be found. 
Kind regards,

Gareth. 

[Steve Q]

I had a servo failure on my c5. Lost all brakes then when I got It recovered by AA it worked fine. No fault codes or warning lights. Had the servo replaced and all good. 

[Stephend161]

9 hours ago, Magnet said:

Hello Stephen, 

Could be backing up wrong trees and older technology-? - servos can be operated by vacuum, so noise originates from loss of vacuum, and if so reason has to be found. 
Kind regards,

Gareth. 

Yes we are hoping to try and replicate the problem again in a few days when i am back up there..

the vacuum pump is something im going to test when i can.

so thanks for your help so far im thinking vacuum and servo are going to be my point of focus 

i will post my findings when i get a chance to get up and have a poke around the car..

Edited October 3, 2022 by Stephend161

[Stephend161]

10 minutes ago, Steve Q said:

I had a servo failure on my c5. Lost all brakes then when I got It recovered by AA it worked fine. No fault codes or warning lights. Had the servo replaced and all good. 

Thanks thats useful to know..

i will be doing that servo brake check test when i get time to get back up there .

we first thought that the only thing that could be making any odd noises the loud squealing type noise that was being heard could possibly be the abs pump as it was the only thing that we could think of that was mechanical enough to make a noice if it went faulty but i have since found out that car has a vacuum pump so I'm going to focus on checking vacuum pump and the brake servo as things seem to be possibly pointing that way at the moment.

 

as soon as i can get back up to my friends land i can do some checks then post my findings here.

so for now thanks to you all for the replies so far if anyone has any more ideas ill be happy to read them and consider them and i will post back my findings in a few days when i get the chance to get properly poking about the audi  

[Steve Q]

You're welcome 🙂 please keep us posted on how you get on 🙂 in my case the issue was so rare it took them a while to replicate the fault as 95% of the time the servo was working as it should. 

[Stevey Y]

21 hours ago, Stephend161 said:

Thanks thats useful to know..

i will be doing that servo brake check test when i get time to get back up there .

we first thought that the only thing that could be making any odd noises the loud squealing type noise that was being heard could possibly be the abs pump as it was the only thing that we could think of that was mechanical enough to make a noice if it went faulty but i have since found out that car has a vacuum pump so I'm going to focus on checking vacuum pump and the brake servo as things seem to be possibly pointing that way at the moment.

 

as soon as i can get back up to my friends land i can do some checks then post my findings here.

so for now thanks to you all for the replies so far if anyone has any more ideas ill be happy to read them and consider them and i will post back my findings in a few days when i get the chance to get properly poking about the audi  

Hi its normally the one way valve in the servo body letting air back into the chamber the whining is probably air moving the wrong way past the valve.

Steve.

[Stephend161]

2 minutes ago, Stevey Y said:

Hi its normally the one way valve in the servo body letting air back into the chamber the whining is probably air moving the wrong way past the valve.

Steve.

Thankyou for the reply things are defo all pointing to one place so far.

ive not had a chance to get up and look yet but when i do defo my findings will be getting posted here..

 

also is the one way valve replaceable or on its own or is it part of something bigger where the whole thing needs replacing .

[Steve Q]

3 hours ago, Stephend161 said:

Thankyou for the reply things are defo all pointing to one place so far.

ive not had a chance to get up and look yet but when i do defo my findings will be getting posted here..

 

also is the one way valve replaceable or on its own or is it part of something bigger where the whole thing needs replacing .

I had the servo replaced with a new one. Best option I think for peace of mind 😊

[Stevey Y]

On 10/4/2022 at 6:45 PM, Stephend161 said:

Thankyou for the reply things are defo all pointing to one place so far.

ive not had a chance to get up and look yet but when i do defo my findings will be getting posted here..

 

also is the one way valve replaceable or on its own or is it part of something bigger where the whole thing needs replacing .

Hi no the valve is replaceable done it before its only held in with a rubber fixing, why would you replace it with a new servo, £££££, you can soak the seal in WD and wiggle it out then pressure test it by using an air line or simply blow down it and then try and suck air through it if you can suck air its probably knackered.

Steve.