S6 C5 brake problems.

[Concker]

Okey, so long story short i have had issues with my brakes for over 2 months now on my 2000 S6
It all started during a track day, suddenly while driving the pedal went soft, and the brakes overheated like crazy to the point smoke was coming out of the wheels.

When back home i replaced the brakefluid with some dot5.1 fluid.
And replaced the front pads with OEM bosch pads.
Now the issues is starting, the pedals is super soft, and basically goes to the floor when the engine is running, (Its fine when its shut off)
So i have flushed atleast 3 liters of fluid true the system using the oldschool pump and hold method, aswell as the local workshop has run true at least 5 liters.
I have also ran 002 command in vagcom and bled the abs pump.

There is no sign of air in the system at all, just clean brand new fluid.
I have also overhauled the calipers with new o-rings and rubber, and they slide easily.
Pistons is smooth and clean, the pads move in and out without problems.

But still the pedal is soft as crazy and the brakes tend to run way to warm to be driven normally in my opinion.
There is no faults codes in the brake system and the abs, esp etc works.

So im guessing next step is to replace the master cylinder?
However i dont see how the cylinder can cause the heat problems...

Any ideas? getting really tired not getting the brakes to work by now..

Thanks.

[Audibot]

What about your flex lines? If you haven't swapped to stainless lines, it may be time--it's possible they are ballooning.

[rollerton]

What about your flex lines? If you haven't swapped to stainless lines, it may be time--it's possible they are ballooning.

What he ^ said.
The inner part of the rubber brake line can corrode and will allow brake fluid to pass through under high pressure from the pedal, but can cause the fluid to be blocked since the piston retracting doesn't provide enough pressure to push the fluid BACK through the line. This can cause the pads to drag and heat up.
Also can cause very soft pedal as the flex lines swell, sometimes not even visibly.
I wouldn't do the master cylinder first, especially if the pedal is firm when the engine isn't running.

[Concker]

What he ^ said.
The inner part of the rubber brake line can corrode and will allow brake fluid to pass through under high pressure from the pedal, but can cause the fluid to be blocked since the piston retracting doesn't provide enough pressure to push the fluid BACK through the line. This can cause the pads to drag and heat up.
Also can cause very soft pedal as the flex lines swell, sometimes not even visibly.
I wouldn't do the master cylinder first, especially if the pedal is firm when the engine isn't running.

Okey, so if the master cylinder is broken the pedal should be soft / smooshy even with the engine off?
I will buy new lines tomorrow as they dont cost much anyways, but still feeling that is not the case (Then again i can be wrong)

[carmudgeon]

My vote is for the master cylinder, although at that age its probably not a bad idea to do the brakes lines anyway. Might as well do them first.

[rollerton]

Well it's not true 100% of the time- but it sounds like the master cyl is working pretty normal, but the master cylinder is so reliable that with your symptoms it doesn't seem as likely?
Do the brakes pull at all? Could be either one, but more likely that brakes dragging wouldn't be the master cylinder from my experience.
And truth is soft pedal is totally common after swapping pads on the C5. I have no idea why, but happens to me all the time and take months to get better. BUt losing pressure isn't.

[Concker]

Hm, yeah and this started before the swap.
I filled up with new brakefluid before the track day, everything was fine.
Then middle of the day the pedal just went to the floor sort of speach, it still brakes but pedal feels very loose.

The o-rings inside the caliper was swollen and the pistons was a pain to get out, however they still looked like new.
So i replaced the o-rings thinking this would fix the heat build up, but no.

Also i checked my neighbors 2.5tdi now with the engine off.
And my pedal seems infact harder and more stiff when i build up pressure. Then on that car.. (where the brakes works perfectly)
So is this a sign that my cylinder may be ok?

So i dont really know whats the problem is.
Still going to buy new lines tho. But need to try to find the correct master cylinder.
When i look at ecs and other website the master cylinder that is listed for my car is wrong.

On that cylinder the 2 biggest plugs when the brakefluid cup is mounted in on top.
On my car the plugs should be towards the passenger side.. (Looks like the 2.7T is the same, just dont know if the diamenters etc is correct, and indeed what part number it should be)
Strange.. =/

The brakes dont pull at all, and im familiar with softer pedal after pad change from my A4 B5.
But that is no where near as soft as my pedal is now. Its basically gripping at the end of the pedal travel.

[carmudgeon]

ETKA lists 2 possibilities:
4D0611021A - tandem master cylinder for vehicles with traction for vehicles with electronic differential lock -edl- control -asr- .
4D0611021B - For vehicles with electronic tandem master cylinder stability program -esp- .

ECS Tuning lists 4D0611021, my 2002 S6 has the electronic differential lock so I would presume that the "A" part is correct.

[Concker]

on ecs if i select s6 c5 it gives me this:

https://www.ecstuning.com/Audi-C5_S6...ster_Cylinder/

Both is wrong, there is no hole for the cup mount, and the holes is pointing upwards.

If i ebay search 4D0611021B

I get the correct one

http://www.ebay.com/itm/100-532-0007...QAAOSwG-1W0idX

So gonna order one of that aswell, i think its the correct one.
Its one of 4 listed on meyle's webpage for this car, that looks indentical, but this is the only one listed to cars with ESP, and it is only up to model years of 01.

[bpark1210]

Just went through this same exact scenario once I rebuilt my rear calipers. My pedal would be firm while bleeding and engine off but once the engine is cranked all pressure drops the pedal to the floor. I'm still under the impression it has something to do w/ the ABS module. Either way I ended up bleeding the damn car like 5 times but re-cycled my fluid since it was brand new and clear each time I bled. Forget what the Bently says about bleed sequence and do a traditional bleed cycle from furthest to closest to the master. I used a pressure bleeder and pumpbed my brakes while under pressure also instead of using the dummy depresser like the Bentley indicates. I have 18z's up front and running stock calipers in the back w/ all new stainless lines. After bleeding the entire system a couple times the pressure will come back. After driving for a few weeks it now seems to have gotten most of the mushy pedal feel out as well as pads are now bedding in for more initial bite.

Check your master cyclinder for obvious signs of leakge before going down the replacement route. You should be able to see the rubber O-rings seeping if the master is the cause of air being introduced.