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Soft brake pedal after AutoX

I've AutoX'd a lot in my M3P and found the stock brakes and pads to be pretty good in this environment. Probably ~200 runs.

This weekend during fun runs, I did 5 runs back to back with less than 90 seconds between them, which is an unusual pace. Didn't even think of it, and just parked the car right after the last run and packed up.

When I got back in the car, the brakes were making creaking noises without touching anything. Just assumed it was stuff shrinking as it cooled off, but this was not a noise I ever heard before.

The instant I touched the brake pedal, I could tell it was soft, with much more travel than before. Brakes still worked, but were different. Pedal was very near the floor before you could get into the ABS.

Drove the car a bit since then, and purposefully triggered the ABS and done brake bedding slowdown cycles. It's recovered about 90% (the ABS really helped), but is still not quite as stiff as before.

Any suggestions what to look at? Did I just de-bed the pads? Boil the brake fluid? Damage a seal? Permanently cook the pads? I'll be doing a brake bleed with some Motul RBF600 to see if that fixes it, which can't hurt against the 2 year old stock fluid anyway.
 
The stock fluid is DOT-3 which is pretty useless in an environment like AutoX.
Not sure I fully agree that DOT-3 is useless in AutoX, given the short runs and inability to build up a lot of heat in the calipers over less than a minute of driving. I never had an issue with 10+ events and 200+ runs even with a 90 MPH max speed at my events. DOT-4's minimum wet boiling point is only 25F warmer than DOT-3, so it's not dramatically better (but race fluids are much better than that DOT-4 minimum for sure).

But your point to bleed is well taken, it will be my first try here. The thing that throws me off is that fluid boil generally fixes itself as the vapor re-condenses, which is why I was asking if anyone had any other thoughts.
 
Not sure I fully agree that DOT-3 is useless in AutoX, given the short runs and inability to build up a lot of heat in the calipers over less than a minute of driving. I never had an issue with 10+ events and 200+ runs even with a 90 MPH max speed at my events. DOT-4's minimum wet boiling point is only 25F warmer than DOT-3, so it's not dramatically better (but race fluids are much better than that DOT-4 minimum for sure).

But your point to bleed is well taken, it will be my first try here. The thing that throws me off is that fluid boil generally fixes itself as the vapor re-condenses, which is why I was asking if anyone had any other thoughts.
You're right - useless is an exaggeration. I'm just honestly surprised you hadn't already flushed the fluid. I think most clubs require a flush within the last six months. A "cheap" fluid like ATE TYP200 has a considerably higher boiling point than the stock fluid. Regardless, that's where I would start, especially considering it's two year old fluid that likely has a bit of moisture in it now.
 
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I'm just honestly surprised you hadn't already flushed the fluid. I think most clubs require a flush within the last six months
Track days for sure. I've never run into an AutoX group that even asked about brake fluid age.
I will feel pretty silly if that's it though ;) I keep up on my brake fluid in my track cars, it is embarrassing if my laziness here was the issue.
 
You boiled your brake fluid and now some air is trapped somewhere in the brake lines.
I generally agree- but the physics of this are weird.
There's no air in the lines. Boiled brake fluid doesn't become air. It becomes vaporized brake fluid.
When this fluid cools down, it becomes liquid brake fluid again, just like boiled water won't stay steam when it cools down.
Generically, fluid fade fixes itself due to this process.
Anyway, time for a brake fluid flush anyway, but I'm only 70% confident this will fix it.
 
Another thing to check for is taper-worn pads. They won't be as obvious and you may have to remove them to inspect, but if they are taper-worn it can cause a similar feel from the brake pedal.
Thanks! Given it happened suddenly and also recovered 90% after a few ABS stops, it probably isn't this, but I will pull the pads for sure when I bleed and check it out.

Gonna bleed/pad check today or tomorrow and I'll report back.
 
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Mash

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Nov 10, 2019
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Recondensing never works 100%. Apparently system is not completely isolated, otherwise why would you flush brake fluid ever. Boiling pushes all brake fluid out and afterwards it recondenses and makes strong vacuum. I don't think that seals are tight enough for strong vacuum, so it gasps some air.
 
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Just to follow this all up since I realized I didn't....

After a week or two and a couple purposeful ABS events, the brakes were 99% again. So recondensing works somewhat.

Ended up busy, and did another AutoX with the same fluid, and it boiled again. This time I didn't let it sit and moved around for 5 minutes, using the brakes lightly, and the pedal came right back.

Changed the fluid, haven't had any issues since, and the pedal is harder than I ever remember it. As long as you bleed from BOTH front bleed screws, not just one ;) Had a real scare there....

I checked the pads and the wear on them was pretty even, and this doesn't appear to be the issue here.

So thanks for all the input, and yeah, you were all correct, this was a simple fluid boil issue, and the stock fluid is OK, but you do need to keep up on it like all cars.
 

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