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.