Failing brake master cylinder?

[M3Convert]

Hi all,



I wanted to get some opinions on my situation and the picture above. I have a dead / soft pedal at the top, say 10-15% of travel. I have bled the brakes probably 4-6 times, including moving from whatever was in the system when I bought the car 3 years ago to a full flush with Castrol SRF. So it has had SRF in it for three years and I've bled it 4-5 times using both a manual bleed procedure as well as with my Motive Power Bleeder. Each time after a bleed it appears to be resolved but then after driving for some period of time it will return. This is especially frustrating because I track the car and no matter what I do, I seem to get a long, soft, pedal on track. Important to note that I am running the Stoptech ST-60 front BBK and ST-40 rear. I have used both Stoptech performance pads as well as Carbotech XP-12/10 combo and I've had the same issue with both pads (getting a soft pedal).

I have used a torque wrench on the bleeders on the calipers and I don't think that is where the issue is coming from.

To try and figure out what is going on, I have inspected the master cylinder and brake booster under the hood (under the cowling) and I don't see anything obvious. I then went under the dash and found that there appeared to be some wetness around the master cylinder rod boot and there was definitely fluid inside the boot. Not enough to be pouring out of the boot though. I wanted to get opinions on what I found and see if this is a definite sign that the master cylinder seal(s) has failed or if I should keep looking for some other cause.

thanks all!

[Ximian]

It should not be leaking, especially that much. Time to replace that booster.

[M3Convert]

Okay, so there shouldn't be any moisture in the boot at all, I assume? As you can see from the picture there is definitely some amount of fluid which I think is brake fluid. I guess I'm ordering a new one from FCPEuro.

[M3Convert]

Originally Posted by Ximian

It should not be leaking, especially that much. Time to replace that booster.

You mention booster, do I need to replace the booster or master cylinder? I thought I'd only need to replace the master cylinder?

[Redd]

Originally Posted by M3Convert

Each time after a bleed it appears to be resolved but then after driving for some period of time it will return. This is especially frustrating because I track the car and no matter what I do, I seem to get a long, soft, pedal on track.

I spent the better part of 2 years chasing identical symptoms. Changed nearly every component of the braking system. Flushed the fluid so many times the insides of my calipers are probably shiny.

In the end my issue was a leaking DSC/ABS pump. There was a slow leak that would only appear when there was high brake pressure - eg track braking. The leak was between the hydraulic and electronic units of the pump. Paint was bubbling there due to corrosive brake fluid leaking out. I'd check the DSC pump if you haven't done so already.

[M3Convert]

Originally Posted by Redd

I spent the better part of 2 years chasing identical symptoms. Changed nearly every component of the braking system. Flushed the fluid so many times the insides of my calipers are probably shiny.

In the end my issue was a leaking DSC/ABS pump. There was a slow leak that would only appear when there was high brake pressure - eg track braking. The leak was between the hydraulic and electronic units of the pump. Paint was bubbling there due to corrosive brake fluid leaking out. I'd check the DSC pump if you haven't done so already.

Food for thought for sure. I should note that the last time I bled the brakes (about a week ago) I did have air in the system mostly from the front calipers. So there is definitely air getting into the system I just can't tell from where.

[Redd]

I found the old thread I created on my brake issues all those years ago. You can have a look at the pics.

https://www.m3post.com/forums/showthread.php?t=1477226

Hope it helps.

[M3Convert]

Originally Posted by Redd

I found the old thread I created on my brake issues all those years ago. You can have a look at the pics.

https://www.m3post.com/forums/showthread.php?t=1477226

Hope it helps.

Thanks for the link Redd. I did just take a good look at my DSC/ABS module and it all looks dry and good.




I was able to use my boroscope to inspect the entire outside of the module / pump and everything looks very dry and pretty clean. Do you think it's possible that it is leaking internally still? I have ordered a used booster and master cylinder as well. Thanks for the input!

[Redd]

While I initially thought it was an internal leak, I don't think it's really a thing. All the cases I've seen of ABS pump failure have looked like mine, with external corrosion.

[DrFerry]

Originally Posted by M3Convert

You mention booster, do I need to replace the booster or master cylinder? I thought I'd only need to replace the master cylinder?

I think Ximian meant to say, "replace the leaking master cylinder".

[Ximian]

Originally Posted by DrFerry

I think Ximian meant to say, "replace the leaking master cylinder".

I've had a smaller leak and I replaced the booster, but did the master cylinder at the same time because you're essentially doing the same work as replacing both. Doing a bench bleed on the master cylinder is going to be a bit challenging.

This was my order from 06/2023 -

34-33-2-283-392 Power Brake Booster $412.08 1 $412.08
34-31-2-283-533 Master Cylinder $527.38 1 $527.38

[bigjae1976]

If the brake booster is leaking brake fluid then it is coming from the master cylinder which should not be leaking.

Replace both.

[M3Convert]

UPDATE: I have not replaced the booster or master cylinder but I have resolved my soft brake pedal. It turns out I needed to bleed the brakes and conduct a DCS/ABS bleed with software. I used Bimmergeeks Protool and was able to get some air out of the ABS pump. I now have a rock solid pedal that I am stoked to try out on track.