A Request to All A6 C4 AEL/AAT Drivers

Hello everyone,

The following problem is what has prompted me to approach you with this request:

I have an A6 C4 TDI with the AEL engine, 140hp.
When I press the brake, for the past few days, it feels like the brake booster isn't working. Then I step on a relatively hard pedal and really have to push down to get it to slow down, albeit poorly.
The behavior is present to varying degrees. Absolutely, but it is ALWAYS present.
When disconnecting the hose from the brake booster, which goes to the vacuum pump, while the machine is stationary, there is a hissing sound. So far, so good... When the machine is running, you can feel the vacuum created by the underpressure pump on the disconnected hose. Unfortunately, I don't know if it's strong enough. Unfortunately, my friend also doesn't have a testing device for that.
If I turn off the engine, the brake pedal becomes hard to press after just three presses. Even if I start the engine while pressing the brake pedal, the pedal usually doesn't sink through.
Do I keep the brake pedal depressed while the machine is running, and does it slowly sink down about 2 – 3 cm? However, not entirely.
Okay, that's that on the symptoms. I also had the error codes read while I was at it. It's clean.

Now, finally, to my request:

Could you please test your own car and see if the behavior I described above is also reproducible in your car?
I currently don't know what is actually defective.
Vacuum pump? Master brake cylinder or brake booster?…..
All 4 brake discs are also gradually darkening... a clear sign that the pads are not gripping properly. But saddles are definitely common.

Does anyone have a tip on what else I could check to track down the error?

Thank you for your effort!!!

Greetings
Wool

Hi,

Can you rule out a leak somewhere on the low-pressure side?

In my 1Z Golf, I once experienced the same symptoms due to a rabbit chewing on the vacuum line. On the AB, I had to really push down on the pedal to get it to work, so much so that the seat was making a really loud noise...

I still need to perform the experiments on my C4 - I don't have time right now.

P.S:
Hm, do the brake discs become dark?
But you're still braking, right?
I mean, you're saying you're compensating for the missing brake pressure with muscle power – but why are the discs then turning dark?

Hi!

The problem can have several causes:

As Markus H. already writes + Air in the braking system, leak in the line, master cylinder has failed, vacuum pump is faulty...

My predecessor (C4) had the main brake cylinder problem (known defects of the C4).

After I had changed it, it was working perfectly again.


Best regards: Marko

Hi Markus,

Yeah... I can definitely rule out any leaks. I just need to check the small 2-way valve that is installed on the lateral drain of the hard plastic hose.

Should I still brake? I have no idea... I need to look at it first...
Yes...yes...I will... *gg*

I'm also not really clear on the issue of brake discs turning black.
That might be because there's a lack of support, and I'm therefore naturally more cautious. And then there's the long distance. I don't need to brake much at all, considering the distance.

I think I'm going to focus on the vacuum chamber again... it seems like the most likely option right now.

But definitely, thank you for your effort!!!

Greetings
Wool

Hi Marko,

I've checked on the leaks so far...except for the aforementioned 2-way valve...

I actually don't really like the idea of using air in the system, because in my opinion, the pedal shouldn't feel very stiff. But I still need to change the fluid. I will then essentially do that in the next few days.

The "main brake cylinder problem" hasn't reached my ears yet....
learned something new...

I will take a closer look at the part after the vacuum pump, if necessary.



So...it was bound to happen...
I just got a call back from an Audi mechanic. He told me about a case on another car that is similar to mine. It was the pump...
It's actually a shame that Audi doesn't have a corresponding testing device for that...


Thank you also for your effort!!!

Greetings
Wool

Hi,

yes, it is allegedly a problem with all 5-cylinder engines (both petrol and diesel).
There are likely leaks, clattering valves (the pump is driven by a camshaft), and so on.
I cannot confirm this personally at this time.

HOWEVER, if the pump internally develops a leak, the oil-pressure relief valve often no longer functions, and oil enters the brake booster system. This is not good! It can also lead to damage to the amplifier itself.
So, better check the pump. You should be able to remove it quite easily. It's conveniently located at the ZylKopf head.

Hi,
I also had this problem in the summer. It disappeared when I replaced the windshield and the door seals. Since they had been in use for a relatively long time (around 100,000 km) and the car had also been unused for more than 4 weeks before, I assumed that the brake calipers might have been stiff in their guides.
That the vacuum accumulator is empty after 2 to 3 brake applications after the engine has been turned off, I have previously considered to be normal, and this was also the case when I bought the car 5 years ago.
However, I can absolutely confirm the clacking pump plunger. Sounds similar to a faulty piston and is particularly annoying, especially at idle with a warm engine.

Greetings from Berlin
Olli

Hi,
For a vacuum, a special measuring device is not required; a vacuum gauge is sufficient, which was also used for adjusting carburetors. Target value?? I once read somewhere about a vacuum of 0.75 bar, which was also read as 0.25 bar absolute.
The main brake cylinder can fail, but it wouldn't have the effects you described. So, either the pump or the brake servo.
The main brake cylinder can be damaged during bleeding/pumping, according to old-fashioned practice, because a ledge can already form at the end of the usual piston rod, which then damages the piston seal. A ventilation system or an adequate DIY solution can prevent this.