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Discussion Starter #1
I recently had the wonderful experience of boiling my brake fluid autocrossing. This has led to a fair amount of reading and pondering my options.

I am aware that my old fluid was 4 years old, so I'm certain that was a major factor. So I replaced the fluid with OEM equivalent, DOT 4LV (Class 6, viscosity 750 or less) for modern ABS brakes. Like all manufacturer warnings, ABS may not operate as intended if I do not use this fluid. However, in light of the realization that the E-differential uses the brakes constantly, boiling fluid seems like the weak link in the stock configuration of the vehicle. I was recommended Motul 660, however that is Class 4 (viscosity 1800 or less) and thus has a higher viscosity. But on the other hand....

Viscosity is most critical in cold weather, which in southern California is not a major problem. I get a 3 to 5 weeks at the most of 30 to 40 F degree weather in my area, and the viscosity test is based on -40! So I'm not certain that regular viscosity would really change how my ABS functions? So, before I make the jump to High performance brake fluid, I want to find some data to clarify this. Personal experience is always valuable, but if you can link a useful website that addresses this directly, like a test or experiment that would be most interesting.

My ABS concerns center around the fact that my car is my daily driver first, and a weekend warrior second. So reliable braking in the real world is my first priority. Remember, real world brakes often operate cold and are formulated for that. Only people who go to a track worry about high temps. So a track set up, may not be optimal for cold emergency brake on the morning commute. And yes, I'm probably over analyzing this, so be gentle. :)

My second question on High Temperature brake fluid has to do with frequency of flushing the fluid. Most websites agree that Hi-temp brake fluid needs to be changed more frequently, but I'm not clear on why. Obviously, frequent heat cycles from track use would break the fluid down, thus needing a flush/replace more often. So that's my primary thought. However, I see two conflicting responses on the internet. Some sources say that Hi-temp fluid absorbs moisture more quickly, but no data is provided. Then to conflict that, nearly every product claims their fluid is formulated to resist moisture absorption and vapor lock. I don't mind changing the fluid, I just want to know why with a logical and scientific explanation. Once again, personal experience is appreciated, but if you can link a good source for some... light reading... I'd like that very much.
 

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... I don't mind changing the fluid,....
If changing the fluid isn’t a concern then why not run an OEM recommended fluid unless you’re at the track?

Why not change the fluid over to a racing fluid on track days?

It would be awesome if you shared the empirical data you find that satisfies your question.

But I agree with your instincts in that you’re over thinking it...especially since your car lives in SoCal.

You can run a Motul 660 like racing fluid year round - just change it every 9-12 months.

If running a racing fluid year round makes you uncomfortable, then change the OEM style fluid to a racing fluid before you track it.
 

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Discussion Starter #3
But I agree with your instincts in that you’re over thinking it...especially since your car lives in SoCal.

You can run a Motul 660 like racing fluid year round - just change it every 9-12 months.

If running a racing fluid year round makes you uncomfortable, then change the OEM style fluid to a racing fluid before you track it.
I think I just want to know the science. Ultimately, I am going to run the OEM right now to see if it will boil again. I'll carry a bleed kit with me this time incase it happens. If it does, then I will switch to the Motul or an equivalent and begin testing on my own if I can't find the answers.

Another interesting issue I discovered is chemisty/solvency. Some online comments indicate that switching types of fluid can cause seals to fail. Yet each manufacturer I am looking at for fluid, claims their product passes rubber compatibility. But there are many rubber formulas depend on car make or year of production, plus whether or not the commentor properly maintained their vehicle. Once again, I'm certain I"m being OCD, but I love knowing "why?"
 

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Discussion Starter #4
So I am not an expert, engineer or anything like that. I'm just an ethusiast trying to figure this stuff out. So having said that...

This is the best article I have found on fluid performance by brand and quality levels. What surprised me was that despite the knowledge that race fluid is very hydroscopic, it was similar to (BMW) OEM fluid. So perhaps it's the bigger influence is the heat cycles of autocross/track use breaking down the fluid faster than daily driving.


Regarding viscosity, I found this article. About half-way down is a chart that show (Oil?) viscosity vs. temperature. Assuming brake fluid is similar, it seems fluid remains very thin until you pass a critical freezing point and then it begins to thicken rapidly. If this is true, for my temperature range running regular DOT 4 will have no impact/change to my ABS system. I will try to find more, but this is what I have so far.

 

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Discussion Starter #5
Here's an engineering article on Heat soak of brake fluid after you park the car from hard braking. At roughly 30 to 60 minutes brake fluid heat soak peaks from being parked, reaching 200 to 350+ degrees depending on variables. The example used is for mountain braking, but for my purpose of autocross it should be the same. This means that OEM brake fluid wet boiling point can easily be exceeded for my second set of runs which I experienced. This also means I might not need High temp brake fluid as long change it more often keeping me closer to the Dry Boiling point. So far I have not made a final conclusion as to what is best for my needs. But it is all very interesting. If only I had an engineer in Fluids and Mechanics to talk too...

 

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Discussion Starter #6
My next research topic is how often High temp brake fluids need to be changed. Some sources say as often as two weeks, or even the same day under heavy use at a high speed track for instance. Others say once a year with street use. Most say depends on brand and use. Additionally, as the fluid heat cycles from hard use it essential becomes cooked and (may) darken. This cooked fluid is more prone to aeration and moisture absorption, I am assuming. I have not posted these sources, because it's a lot of reading and many are secondary quotes that give the original source for verification.

So I did a search for Motul, as that is one brand I am interested. I found this vendor explanation, comparing Motul 5.1 to 600. For the record, vendors are trying sell stuff, so I generally assume they are painting the product in the best light. My advice, search your brand directly with key words like "moisture absorption". And be prepared to sort through the many vendors that offer no useful answers on real world application. And take everything on the internet, including my posts, with a bit of caution as they could be misinformation, biased or out of date. Please add to this thread if you have good information, a good online source or personal experience with specific conditions your can describe.

 

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Discussion Starter #7
So I just ordered Motul RBF 600 and plan to bleed the brakes and clutch after the holidays for my next autocross. Once done, I will "test" the ABS on a cold morning, note the temperature and how the ABS responds. I am predicting, since the lowest the temperature we get here is around 32 F, ABS will operate fine for my needs.

My long term test I suppose should be those test stripes and take monthly readings of the water content in the Motul.
 

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interested in results of water absorption..
 

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Discussion Starter #9
Update: I bleed the brakes (and clutch) today with motul 600. I then went for a test drive. Brakes feel like I'm getting more pedal feedback. Hard to explain, but I could briefly feel the rotor passing across the pad while braking around a corner at low speed. ABS seems to work. 68 F, so it is warm. But it does feel different. With the low viscosity it's like a bunch of little pulses. With the Motul, it's like several larger pulses. Does it work well? The candy wrapper in the back seat that flew past me out the window when the ABS went into action definitely surprised me! But I'm not sure that's "objective science."

The real tests are as follows:

1) Cold mornings? Will ABS work correctly down to 32 F, the coldest it gets in my area. I should have an answer by the end of February.
2) Will it damage the ABS unit? Only time will tell. I'm thinking if it did, someone would have complained by now.
3) Will the 600 noticeable help in autocross, beyond thermal resistance? I'll report back after Sunday. :)
4) Will the 600 contaminate faster than regular brake fluid, with once a month autocross? I need to order some test strips and begin testing monthly to data record.

As a side note regarding bleeding the clutch. Yes the clutch on this car can be tricky to bleed. But this was my second system bleed and last time I did the brakes only. Contaminants were being siphoned from the clutch into the brake system. The clutch fluid was icky! So if you are up to it, I recommend bleeding the clutch. That being said, many have posted having difficulty doing it, so do your research. I used a Motive Pressure Bleeder and topped off the reservoir before I began. I believe Ford designed the system for a reverse pressure bleed which is why so many people get air in the clutch lines. But now I'm off topic on my own thread...
 

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Discussion Starter #10
Autocross update: Brakes felt great today! When it was cold, mid-40's, the stock brakes felt grabby on the first run, like when the car was new. During the rest of the runs the brakes worked consistently and dependably, with no significant tapper off with each run. There was one run I thought the brakes were letting me down, but then I realized that I had left TC on, so that was my fault. The following run was my best time of the day once I turned the TC off. ;)

ABS worked fine from mid-40's up to upper-60's just fine with no problems. All speeds were 2nd gear. ABS is more noticeable now, as the pulses feel more assertive.
 

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Good to know! (y)
 
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