Is the master brake cylinder going bad?

[CyberPMG]

About a month ago, I had swapped out the old BBK with a new BBK (also replaced rear rotors and new brake pads). Did a full flush of the brake fluid. Everything was buttoned up, pads properly bedded, and everything has been working fine. Did a trip to the Smoky Mountains where there was lots of braking on the tight twisty mountain roads (The Tail of the Dragon). That was 3 weeks ago.

Last weekend, I started to get the brake light to start flashing. Found out I was suddenly low on brake fluid. Bought some more brake fluid (Motul RBF600) and had to use 1/3 of a bottle (500mL bottle) to bring the fluid level back up to full. Took it to my local shop to have a look. He checked the slave cylinder and clutch line. They are bone dry. I'm not seeing any brake fluid on the ground. So where's it going?

My mechanic thinks the problem may be the master cylinder. He started the car then firmly stepped on the pedal. After about 5 minutes of constant pressure, the brake pedal will move all the way down to the floor. He thinks because of this, there must be a leak within the master cylinder which will cause the brake fluid to leak out into the brake booster.

Does this make sense? I've heard how difficult it is to pull out and replace the master cylinder, so I want to be sure that's the right thing to do. If so, I have another question. I looked on ECS Tuning's website and saw that there are 2 types of master brake cylinders - one for a TRW brake system and one for an ATE system. How do I know which one I have? Does it matter?

I know I'm not going to upgrade to a S4 master cylinder since that causes all sorts of problems, but I'm wondering if there are any upgrades like there have been for the slave cylinder (metal instead of plastic)... or should I simply stick with OEM (since the one I have came with the car and lasted 8 years and 229k miles)?

Finally, if replacing the master cylinder, should I go ahead and replace the slave cylinder with the upgraded metal version (the USP Motorsports is offering)? The slave cylinder was replaced once when the clutch was upgraded (some 180k miles ago). It's still working fine with no signs of leaking. Same is true for the clutch line.

Opinions are appreciated!

[diagnosticator]

Yeah, the brake pedal should not leak down like that. If brake fluid is leaking into the booster, from the MC, it will get sucked into the intake manifold and burned. What are the problems you mention with the S4 master cylinder? The only aspect I know of is that the push rod length must me set exactly right, but the Bentley does not give different specs between the push rod lengths for the S4 vs A4. The larger diameter S4 MC will provide for less pedal travel compared to the A4 MC used with a BBC.

[Duchenbagen]

The S4 master cylinder causes havoc with the ABS pump. IIRC jet jockey tried this upgrade and ended up reverting back to the A4 master cylinder.

[MikTip]

The only difference between the A4 MC and the S4 MC is the A4 is 25mm bore size and the S4 is 27mm....

Not sure what Jet Jockey did or specifically what S4 MC he had....

[MikTip]

I just replaced my brake MC on my 2004 A4.

If you look at the current MC on your Audi you can see its make(ATE/Girling/TRW) and get the correct replacement.

I bought a replacement from Centric Brake(130.33116). It fits and functions perfectly!

[boy412]

The S4 master cylinder causes havoc with the ABS pump. IIRC jet jockey tried this upgrade and ended up reverting back to the A4 master cylinder.

Yes...something weird about re-coding the ABS pump. Sounded like a LOT more trouble than its worth...and our cars are a big enough PITA as it is sometimes.

My MC failed when I had a fluid leak at one of my rear calipers. It happend on a road trip and I ended up limping it home. No amount of bleeding could get all of the air out...so the MC ended up getting replaced. No problems since.

[diagnosticator]

The S4 master cylinder causes havoc with the ABS pump. IIRC jet jockey tried this upgrade and ended up reverting back to the A4 master cylinder.

Sounds like something else was wrong, since the A4 and the S4 use the same ABS unit.

[diagnosticator]

The only difference between the A4 MC and the S4 MC is the A4 is 25mm bore size and the S4 is 27mm....

Not sure what Jet Jockey did or specifically what S4 MC he had....

2nd. If the S4 MC that Jet Jockey used was a total no go, then something else must have been involved.

[MikTip]

2nd. If the S4 MC that Jet Jockey used was a total no go, then something else must have been involved.

I know the A4 has 3 different MC's... 2 are interchangable, 1 is not....
The S4 appears to have 1 MC, which apprears to be identical to the 2 A4 versions, except for the 27mm bore size.
Perhaps JJ had the on MC with the different push-rod length?

I was going to swap in the closest looking S4 MC, but didnt feel like it would gain me very much compaired to the OE A4 MC.
What can 2mm really gain? Slight less pedal travel? Slight higher output pressure/volume?

The Centric TRW replacement MC I bought and installed works just fine! Even with my 365mm 8piston caliper RS4 brakes!

[muizenissen]

I know the A4 has 3 different MC's... 2 are interchangable, 1 is not....
The S4 appears to have 1 MC, which apprears to be identical to the 2 A4 versions, except for the 27mm bore size.
Perhaps JJ had the on MC with the different push-rod length?

I was going to swap in the closest looking S4 MC, but didnt feel like it would gain me very much compaired to the OE A4 MC.
What can 2mm really gain? Slight less pedal travel? Slight higher output pressure/volume?

The Centric TRW replacement MC I bought and installed works just fine! Even with my 365mm 8piston caliper RS4 brakes!

This quite a thread-bump (almost a year old :D) - still wanted to ask the question though. Pardon my intrusion - I haven't made an intro for myself yet (will do at some point), so just a bit of background in this post.

I've been looking for a solution for my problem with my upgraded brakes. I'm running the Cayenne/Q7 6-pot 350mm setup on my 2004.08 A4 (Dutch car :) ), and although they work perfectly, I still get the feeling that I have to push my pedal quite hard for them to actually work (and thus - more often than not, the brakes don't really 'bite' because they simply don't get warm enough). Sometimes, even, it feels like pressing the brake pedal gently results in the pads being lifted (as in - the car doesn't brake) and you have to brake much harder (almot as if your pads are all glassed out).

Now - some of my local garagists told me that this was likely due to the 25.4mm MC, and the fact that fluid simply doesn't flow fast enough to take full advantage of these calipers (which were to be used on a dual stage setup). Swapping the MC for an S4 one wouldn't work, as that's a 26.9mm dual stage version as well, and that doesn't fit.

Is there anything else I can try? All newer Audi's seem to have the dual stage setup, so that's out of the question I guess. Additionally, I'm affraid that upgrading the MC will mess up the ABS coding / expectations (although the brakes now work properly when pushed hard on the pedal, and are very well controllable, and ABS rarely kicks in even though braking is very consistently and hard).

Anyone got any tips?

I'm quoting MikTip, because he's using the Centric Brake part, which is also a 25.4mm part. The 8-pot RS4's seem to be even bigger than the Porsches, so I'm curious to his experience on the 25.4mm device..