Hello out there! I am new to the forum and have attempted to look up information on this topic but never seem to find a solution. Here is a quick background on my current issue and what I'm trying to accomplish:

I have an 06 MCS and have recently installed a TCE Performance Wilwood Big Brake Kit (4 piston) up front. I have not touched the rear brakes yet. I used Motul RBF600 Dot 4 Brake fluid. After installation of the BBK I started the bleeding process using a Power Bleeder kit. Things went well and then I went for a test drive. I started the bedding process and noticed I had a spongy pedal still. I then took the car to a BMW specialist and they were able to bleed the ABS unit. They did a great job and the pedal was firmer. The pedal travel was a little further than what I had with the stock front brake setup but still firm. After completing the bedding process I attended a local AutoX event and things got interesting...

A few warm up laps in I began to push the car. On the very last 2 turns I would brake hard, the pedal would be firm with plenty of bite, and then out of nowhere the pedal would travel to the floor and I'd have little to no stopping. Luckily I'm not going fast enough to be in any danger but I know brakes don't normally give out like that. After some additional bleeding and test drives still the same result when I come to a hard stop from 50+ mph.

Troubleshooting leads me to believe it's the Master Cylinder failing under that much pressure and fluid is bypassing the seals.

With all that said, I was looking into a stronger master cylinder setup to give me a better pedal feel and quicker bite. Has anyone attempted to swap an entire R56 S Master Cylinder and Booster assembly into an R53 S? Both complete setups look pretty close in configuration. From what I understand, the R56 S uses a Vacuum pump off the engine as an assist. I could probably hook up an electric vacuum pump to compensate for not having that on my R53.

Top Pic is R56 assembly
Bottom is an R53 Assembly
Right is BBK installed

 

Going to move you to the Stock Problems/Issues area.

And welcome to NAM.

 

Thank you!

 

Do you know the size of both? I know the r56 jcw brakes are an upgrade, but I can't imagine they put a much larger master cylinder in such a similar system. Disclaimer: I have no idea - just playing devil's advocate.

 

I just looked into all of this a little further the last few days... so it looks like it can be done but there is no port in the r56 master cylinder for the DSC sensor which I don't want to tamper with since it is still a street car. That being said, I've decided to just replace mine with another r53 oem MC. Hopefully it holds pressure once replaced. I noticed brake fluid had leaked out of my current MC onto the booster and the black paint is peeling off so theres my sign. :-) When my MCS becomes a track car one day I plan to to swap the stock assembly for a GP2 MC and booster setup. If all goes well I will post results. Thanks again!

 

Thread revival - is there an update here? Was the replacement master cylinder a success, or did you try R56 master cylinder and brake booster?

 

Also did you check the hardline which goes to the brake booster?

 

Everything has been checked.

 

Hey, good morning! I ended up installing a new stock master cylinder/brake booster. I've been to multiple track days with it and everything is working great so far. I've upgraded to H compound pads on my Wilwood brake kit which have a lot more bite. I also installed a sneed4speed brake cooling kit. Big difference at the track last Sunday. I had strong brakes for multiple 20 minute sessions versus past track days where I needed to do a cool down lap mid session. Pleased with everything so far. Thanks!