Stiff brake pedal

[frontoya]

Recently I noticed on 2 separate occasions that when I get in the car in the morning, the brake pedal is very stiff. As soon as I start the engine, it immediately softens and goes to the normal state. Once it was parked overnight, and the other time over the weekend (3 nights). No braking issues, the brake fluid level is normal, no leaks under the car. It doesn't happen every time. The car is parked on a level ground (garage) and I don't engage the parking brake.

Googling shows that there this exact problem with other Audi cars, and they even issued a service bulletin:
https://static.nhtsa.gov/odi/tsbs/2021/MC-10187578-0001.pdf

But RS3 is not listed as affected, and the part being replaced doesn't appear to be in RS3 -- part number doesn't fit.

Any ideas what it could be?

Prior to this I've installed 4" intake and elbow (CTS) and bigger intercooler (CTS), but it went smoothly, and I triple-checked that the intake pipe don't press/rub/interfere with any wires/plugs/tubes/etc.. Also, did DS1 tune (stage 1), but that's probably even less to do with brakes. The only thing I can think of is that I've done a good amount of spirited driving after the tune, and the car doesn't always have a chance to completely cool down before I park it. Could that be the cause?

Should I just replace the brake vacuum hose that goes to the booster? For some reason I can't seem find the corresponding part number for RS3. Looking under the hood, it has silver heat shielding and doesn't appear to have a sensor? It doesn't seem to match the picture for A3 or S3 hose. Every brake vacuum hose I found on Audi parts website says it doesn't fit RS3. Any ideas how to locate the part number?

Thanks!

[frontoya]

Figured I document my search here, in case someone else encounters the same problem.

Decided to replace the brake vacuum hose as per the service bulletin I linked above, even though it doesn't specifically reference RS3. For the life of me couldn't find the part number. Searching various OE parts websites for brake vacuum hose for RS3 didn't return anything. Same for the official Audi Parts website.

You can find the power brake booster for RS3, it has a diagram that shows the hose I need, but the parts listed below the diagram are all screwed up, with the numbers not matching the description for a lot of them:
https://parts.audiusa.com/a/Audi_2018_RS3/76193973__6698770/Cowl-Components-on-dash-panel/1312530.html

The hose is labeled #11 on the diagram above, but the parts list calls it a gasket. There is absolutely no hoses on the lists whatsoever.

Drove to the local dealership to figure this out. They insisted this is the part number for the hose for RS3 based on my VIN:
https://parts.audiusa.com/p/Audi_2018_RS3/Brake-Vacuum-Hose/70168678/8V0612041A.html

I already came across this part and already knew that the official Audi parts website says it does NOT fit RS3, and is for TT RS. The parts guys at the dealership couldn't explain it.

Additionally, looking at the diagram, it shows that the hose has a pressure sensor at the booster end. I invited him to take a look at my car to see that my hose does NOT have any sensors. He called out a tech, and they looked at the hose in my car, and had to conclude that there is no sensor. They then called the technical assistance line at Audi, came back and just repeated that the 8V0-612-041-A is the correct part number. The parts guy, looking at my after-market filter and intake suggested that maybe I installed a different hose myself. Obviously nonsense, simply because if I installed a hose without a sensor, I would have a CEL lit up, and why would I even come to them if I already knew what hose I needed. In any case the dealership was completely useless.

I'm thinking I'll just get the part with the sensor, and leave it unplugged.

I'm a bit pissed at Audi that the car came from the factory without a sensor that's supposed to be there by all accounts. It's part of the safety check, isn't it? Curious if anyone with a 2018, 2019, etc. RS3's can look at their brake booster and see if there is a sensor on the vacuum hose, or not? Maybe the earlier year cars came without the sensor, and later ones with?

[Rod Paine]

Interesting problem. We have a 2019 RS 3 and now at 47,853 miles of which 34,390 miles have been fast country back road
runs made 3 days a week as weather permits with no brake pedal problems at all. Their doesn't appear to be anything like
a 'sensor' attached to our brake lines.

[redline380]

I'd say the likely culprit is a small vacuum leak. Going hard over night means the brake booster is not retaining the vacuum. The booster itself may be leaking.

I would start with a vacuum leak test.

[willbardinb5s4]

Figured I document my search here, in case someone else encounters the same problem.

Decided to replace the brake vacuum hose as per the service bulletin I linked above, even though it doesn't specifically reference RS3. For the life of me couldn't find the part number. Searching various OE parts websites for brake vacuum hose for RS3 didn't return anything. Same for the official Audi Parts website.

You can find the power brake booster for RS3, it has a diagram that shows the hose I need, but the parts listed below the diagram are all screwed up, with the numbers not matching the description for a lot of them:
https://parts.audiusa.com/a/Audi_2018_RS3/76193973__6698770/Cowl-Components-on-dash-panel/1312530.html

The hose is labeled #11 on the diagram above, but the parts list calls it a gasket. There is absolutely no hoses on the lists whatsoever.

Drove to the local dealership to figure this out. They insisted this is the part number for the hose for RS3 based on my VIN:
https://parts.audiusa.com/p/Audi_2018_RS3/Brake-Vacuum-Hose/70168678/8V0612041A.html

I already came across this part and already knew that the official Audi parts website says it does NOT fit RS3, and is for TT RS. The parts guys at the dealership couldn't explain it.

Additionally, looking at the diagram, it shows that the hose has a pressure sensor at the booster end. I invited him to take a look at my car to see that my hose does NOT have any sensors. He called out a tech, and they looked at the hose in my car, and had to conclude that there is no sensor. They then called the technical assistance line at Audi, came back and just repeated that the 8V0-612-041-A is the correct part number. The parts guy, looking at my after-market filter and intake suggested that maybe I installed a different hose myself. Obviously nonsense, simply because if I installed a hose without a sensor, I would have a CEL lit up, and why would I even come to them if I already knew what hose I needed. In any case the dealership was completely useless.

I'm thinking I'll just get the part with the sensor, and leave it unplugged.

I'm a bit pissed at Audi that the car came from the factory without a sensor that's supposed to be there by all accounts. It's part of the safety check, isn't it? Curious if anyone with a 2018, 2019, etc. RS3's can look at their brake booster and see if there is a sensor on the vacuum hose, or not? Maybe the earlier year cars came without the sensor, and later ones with?

Wondering if this fixed your issue? I get the hard brake pedal as well, but I go into limp mode and get a drive train malfunction warning on my car. 2019 RS3 with 66,000 miles here.


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