Hard Brakes, strange issue

[raceyBMW]

Went to the track today with my 2011 M3, recently swapped my race pads back in for the track day and bled brake fluid.

First lap or two brakes feel great, but then the pedal gets very very stiff, like where it feels there are no power brakes. Car does slow down but I have to give the brake pedal way more foot pressure than I have ever had to before. Am I looking at a potential brake booster issue? I haven't gotten back under there yet to look, but a bad bleed makes brakes mushy, not hard.

I've never dealt with this issue before now, and I have used this setup a couple times before.

Thoughts?

Pic for attention

[roastbeef]

odd issue. is it slowing like normal, or just much more pedal pressure is needed? have you ran that fluid before?

[Redd]

Insufficient brake vacuum will make the pedal hard. So bad booster or brake vacuum pump?

[raceyBMW]

Originally Posted by roastbeef

odd issue. is it slowing like normal, or just much more pedal pressure is needed? have you ran that fluid before?

Yes I have, it's just Motul RBF660. It does slow, but really need to get the leg in it, nothing I have ever experienced before and I have been running race pads for a handful of track days now.

Originally Posted by Redd

Insufficient brake vacuum will make the pedal hard. So bad booster or brake vacuum pump?

Yeah, I was doing some more research and sounds like a bad brake booster sensor is semi-common, especially if it has had brake fluid on it in the past. I'm sure I've compressed the caliper making it overflow before, so could the culprit.

[roastbeef]

I had the vacuum (not sure exactly what component, it was under warranty) go bad and it caused limp mode. Not sure if that helps...

[Bartledoo]

Originally Posted by raceyBMW

Yeah, I was doing some more research and sounds like a bad brake booster sensor is semi-common, especially if it has had brake fluid on it in the past. I'm sure I've compressed the caliper making it overflow before, so could the culprit.

I can't speak for all cases, but I've had the sensor failure due to brake fluid frying it, but this would just cause the vacuum pump to run a ton as a fail-safe means for supplying proper vacuum boost. In other words, there was never an issue with inadequate brake boost with a failed sensor, in my case.

[Redd]

Brake booster sensor failure causes constant running of the vacuum pump and eventually results in errors and limp mode. No hard brake pedal when this happens.

[raceyBMW]

Originally Posted by Redd

Brake booster sensor failure causes constant running of the vacuum pump and eventually results in errors and limp mode. No hard brake pedal when this happens.

Hmm, Ok. Was hoping it was easy, but sounds like I need to keep digging.

Thanks for the comments, keep them coming if you have experienced anything like this before.

[bigjae1976]

Could be a bad booster or a bad solenoid. I believe there is a solenoid operated valve which routes back to the intake. It builds vacuum on the booster by closing, releases by opening.

If there were a leak, you should get a code for lean condition.

[JP10]

OP, did you every come to a resolution on this? Coming from the Gym into work this morning I noticed that my brake pedal was more difficult to depress. Maybe I'll try pulling the codes after work to see if I have anything that the others have mentioned.