High water % in brake fluid resivoir. Thoughts?
 

Technically this isn't related to my BRZ, but I've seen no responses in my BMW forums so I thought I'd see what general automotive insight you folks might have.

Any ideas on what's causing the water? Any suggestions for solving the problem?

I have 4%+ water in my brake fluid according to two ~$10 testers. Those same testers indicate 0% on fresh from the can brake fluid, so I'm inclined to trust the readings. I had similarly high readings before I completely bled the system about 6 months ago in Oct '19. The vehicle functions normally, but it (an X5M) is really just a daily driver so I'm not really hammering on the brakes like I would on a track car. There are no leaks, and the fluid is a decent amber color. A test strip suggests 0% copper, so I don't have obvious indications of internal metal deterioration. The only sign of issues was last Oct when one rear bleeder screw was corroded to a point where fluid wouldn't pass out the nipple- I bled the caliper by unscrewing the valve enough to drain from the screw itself.

Thanks to anyone who has any ideas!

How often are you taking your brake fluid res cap on/off? Have you tried replacing the cap and seal yet? What kind of fluid are you putting into the system?
I know you say you aren't hammering on the brakes, but how frequently are you getting them hot?

Cap wasn't off for the 6 months in between. I've never replaced it- presumably it's original to the vehicle (late 2010 build of a 2011 MY)

Fluid is Pentosin Super DOT 4. The vehicle ought to get the low viscosity stuff, but I didn't realize the error until I was already done with the job.

Hot brake frequency? Possibly never, perhaps rarely.

My thoughts are you need to do a flush and fill. And more often.

Agreed...to a point. There's nothing normal about seeing such water levels 6 months after flushing and filling with fresh, run of the mill fluid. I'm trying to address the underlying problem and already ordered a new reservoir cap based on the initial question/suggestion.

well brake fluid is hydroscopic, meaning it pulls water from the environment.

as far as what percentage is good/bad, i really don't know...

0% = Ideal
1-2% = OK, pretty typical for old fluid in the first year or two.
~3% = Time to change, preferably soon.
4%+ = Change it now. You're risking brake performance from overheating/boiling the fluid and from corrosion within the calipers/lines.

Check that the expanding rubber seal is in tacked. Evan a small hole will let air in and out with a change in barometric pressure. Over that period of time did the rubber seal follow the level down as the brake pads were wearing out?
I also agree that you should be changing the fluid more often, now you know there is a problem.


Not sure what method of testing you are using but it may be telling you percentage of water that the fluid can hold dissolved rather than percentage of the fluid by volume.

Check the Cap seals. Bleed the brakes every year at a minimum. It’s not that hard to do.

Maybe I'm reading this idea the wrong way, but I've seen similar ones in other replies too: "Just bleed them more often."
That's fine, and my plan to a point. The idea a system might require a full bleed every 3 months is just nuts to me. Maybe the cap is the primary issue. I hope so.
Am I reading the overall tone right that this isn't really a big deal to anyone else?

Well, if you want to do it up right, I suggest you change out the master cylinder, the slave cylinder and the line that runs between the two, then bleed the system.

After that, if your test still show water in the system, throw out your test equipment.

There ya go - ;)

How do the brakes feel? After bleeding I have never felt a difference in pedal feel. But I have never had contaminated brake fluid that I know of.

Some who track that have really overcooked their brakes can feel a difference after a bleed.

If you are feeling significantly different brake feel after a bleed then I would certainly start inspecting everything. If not just bleed every 6 months and forget about it.

They felt fine before the bleed last fall & essentially the same after. They still feel fine today. I definitely know the feeling when going from track-cooked fluid back to fresh like you're talking about. The lack of it here is part of what makes it so weird for me given the water present.


I'll just take the route of bleeding them more frequently (+ replacing that reservoir cap). My first X5 I had ~15 years ago had an issue with the brake lines failing and needing to be replaced. Presumably that was from corrosion, but it's not like I'll ever know exactly why they sh1t the bed in a parking lot. Maybe I'm just gun shy as a result.

Are you using this type of DOT 4 brake fluid tester to test for moisture content? https://www.amazon.com/PTE-Tester-Ca...005HVG4GQ?th=1
The most accurate way of determining the water content in brake fluid is to have it tested by a garage that has a boiling point tester.
The main flaw with the cheap conductivity testers is that these testers will estimate the water content electronically by measuring the conductivity of the brake fluid, which in theory increases as water is absorbed. The tester then converts this measurement to a supposed boiling point and indicates the result using an algorithm as a display of green, yellow or red lights as % water, depending on the conductivity. Unfortunately, this measurement principle is fatally flawed as the conductivity of new brake fluids varies substantially from formulation to formulation within a DOT grade depending on the additives used. This means that unless a conductivity tester is calibrated on one manufacturer’s product and then used for that product only, it is likely to give very inaccurate results.

The maintenance schedule for my FR-S states to change the brake fluid every 30K miles (Normal), or 15K/15 months (Severe). My FR-S is 7+ years old but only has 28K miles on it. I recently had the brake fluid tested with a boiling point tester. Its moisture content was 1%. I suggest you do the same.

Fair point. It is a cheap one like you linked. Technically, it’s two of them from different manufacturers because I happen to own a spare. For what it’s worth, a fresh bottle of the same fluid reads 0%.

https://www.bendix.com.au/bendix-new...-fluid-testing

"Bendix does not recommend using a conductivity tester as they have been known to fail when testing freshly opened brake fluid. Conductivity testers estimate the moisture content by converting the conductivity into an estimated boiling point. Since brake fluids vary from formula to formula, this test is not an accurate result unless the tester is calibrated to the manufacturer’s product when new.

Bendix recommends a tester that heats the brake fluid to determine the correct boiling point. Using a boiling point tester is the only accurate way to test brake fluid performance."

Since you do not know what brake fluid your conductivity testers were calibrated to there is no way of knowing if the results you are getting are accurate.

OK. I'm convinced my cheap-o tester(s) are likely worse than simply assessing based on known age, appearance, and brake performance. Ive had enough strange results over the years: better off simply not using them.

I just sprung for a real boiling point measurement tool. On the off chance folks are curious what a real measurement produces, I'll update once I have better data in a week or so.

Thanks everyone!

One thing I would like to point out here is that your ABS system may also need to be flushed in order to remove all of the fluid in the system. A simple bleeding of the system at each wheel will NOT flush whatever is inside the ABS module. For that, you need to have a diagnostic tool that is two-way and able to turn on and off each of the individual solenoids inside the module.

And since it’s a BMW, there is probably a lengthy procedure to accomplish this. You will probably never get all of the old fluid out if you don’t go through this procedure which may result in there still being moisture in the system when you go to test for it.

I agree but after 17 years on my WRX, I have had no issues with just bleeding the corners to get the fluid out of the caliper. ABS still works, check it every winter in the snow.

We’re talking two different vehicle manufacturers here. BMW are NOTORIOUS for making things difficult for the home mechanic. I’d be willing to bet that their ABS system is completely different in design and requires completely different maintenance procedures from that of a 17 year-old Subaru WRX.

Somehow the OP got moisture in his fluid at some point. That fluid is pumped through the system by the ABS pump inside the module.

Think of it like draining your automatic transmission fluid. You’ll get most of the old, dirty fluid out of the pan, but you still have all of that old, dirty fluid in your torque converter left when you’re done. Unless you’ve completely flushed the fluid out using the proper procedure, the remaining fluid trapped in the torque converter will eventually mix with the new fluid that you’ve added to the tranny to compensate for what you drained out of it.

Here’s a story: When I helped my son change his transmission fluid in his Scion tC, instead of dropping the pan and just draining it out, we pumped it out using the transmission cooler high pressure line up by the radiator just by idling his engine and shifting the tranny in and out of all the forward gears and reverse gear. Once the flow slowed down enough to let us know it was empty, (and without running the pump completely dry) we would shut off the engine then pump new fluid back in on the return hose using a drill-operated fluid pump until it showed on the dipstick.

We repeated this procedure several times until the color of the fluid coming out of the high pressure hose looked like the new fluid that we were pumping into it. It took several hours and about four gallons as we were doing this in our garage at home and we were using makeshift tools. It was very messy, and I would hate to have to do it again, but at least we walked away knowing that the job was done properly.

I then had him take his car to a local shop the next day to install a new pan filter kit that he had purchased and top it off using the exact same fluid we used that they drained out during that process.

So basically, we performed the difficult and messy part that none of the local shops around here would do. And yes, we called several different shops and none of them would perform a complete tranny flush like we did.

My point for bringing up this story is that had we not performed a complete flush of his transmission, the nasty burned fluid that was left over in there had we just let a shop perform a standard “transmission service” would have just mixed with the couple of quarts of new fluid. Short of pulling the entire transmission, completely disassembling it, physically cleaning it all out, reassembling and reinstalling it, this was the next best thing we could have done.

Older cars with brake systems that don’t have ABS are simple and flushing the system is as easy as bleeding the lines at all four corners topping the master cylinder off as you go until the fluid is nice and clean. But cars nowadays just AREN’T that simple. You just won’t get all of the brake fluid out unless you follow the correct procedure.

Yeah I am not disagreeing with you. But I imagine a large percentage of BMW owners never do this and the brakes work just fine. That said, enthusiasts like us want things working their best, so I could see myself or others trying to do it correctly.

Agreed. However, the OP’s original problem wasn’t that his brakes weren’t working properly, it was a complaint of moisture in his fluid which would come back after bleeding them. That’s what I was attempting to address and offer a solution to his particular problem.

With the Farcom DF.16 tester, I got a boiling point of 182 C.
Fresh fluid which is essentially the same, reads 250 C.
The plan remains the same: bleed and replace the cap. Check again in 6 months.

If you are using Pentosin Super DOT 4 brake fluid it has, per specs, a BP of 265°C and a wet BP of 165°C. If your brake fluid is reading a BP of 182°C, then yeah time to change it. http://www.pentosin.net/specsheets/P...SuperDot_4.pdf
Never heard of a Farcom DF.16 tester. Misspelled, should be Facom. You spent some bucks, even used not cheap. https://www.ebay.com/itm/FACOM-DOT-T...75.c100623.m-1