[ctcarbonari]

Originally Posted by XutvJet

Lastly, I've tested 4 y/o brake fluid on my M235 at the reservoir and at the caliper. Hardly any measurable difference and within 0.5% of a percent, thus, so much for that argument and it really opened my eyes to how needless a 2 year fluid change is on a street driven only car.

I always wonder about the difference of water content (reservoir vs caliper).
Nice to see someone who has posted empirical data.

[Redd]

Originally Posted by XutvJet

Lastly, I've tested 4 y/o brake fluid on my M235 at the reservoir and at the caliper. Hardly any measurable difference and within 0.5% of a percent.. .

Within 0.5% at both locations (reservoir & caliper) or within 0.5% as compared to fresh brake fluid?

[Grease23]

Originally Posted by ctcarbonari

I always wonder about the difference of water content (reservoir vs caliper).
Nice to see someone who has posted empirical data.

Diffusion is your friend.

[Redd]

Originally Posted by Grease23

Diffusion is your friend.

LOL yeah. I would not expect much difference between reservoir and caliper since it's the same system.

I would be very surprised if it was only 0.5% difference in moisture content as compared to fresh brake fluid tho.

[JABCAT]

Originally Posted by XutvJet

The German makes are just bonkers about their brake fluid changes but if you go to an Asian or domestic shop, they hardly ever recommend a brake fluid change unless it's been years, the car really old, and/or the fluid in the reservoir brownish. Those cars use DOT 3 or 4.

Quick search shows:
Subaru: 30 months/30k mi brake fluid replacement.

Acura: 3 yrs regardless of miles

Honda: 3 yrs/ 30k mi

Lexus: 3 yrs/ 30k mi

[XutvJet]

Originally Posted by Redd

Within 0.5% at both locations (reservoir & caliper) or within 0.5% as compared to fresh brake fluid?

Aged fluid. I was like 1% at the reservoir and 1.0-1.5% at each caliper. The back ones were ~1.5% and the fronts ~1%.

[RockCrusher]

Originally Posted by XutvJet

Unless you are tracking a car, the recommended brake fluid changes in German cars is extreme.

I have a brake fluid tester and I have yet to find a fluid that doesn't go out of spec (i.e., 2%+ moisture) until around 4 to 5 years. My M235's 4 y/o fluid still tested at just under 2% when I sold it. My daughter's 2015 Outback, which we've owned since new, didn't exceed 2% until year 7.

I've been driving since 1989 and have done all my maintenance for as long as I can remember. Not until I got my new 2016 M235 back in late 2015 did I bother with brake fluid changes.

I've NEVER had a brake related issue in the 15+ cars I've owned between me, my wife, and kids. No fade, no degraded/fail brake lines, no ABS issues, etc.

I'm not saying don't routinely change your brake fluid, but for most cars, every 2 years is unnecessary and is just a money grab and another way to get a customer into a shop, put some fright into them because they are "brakes", and upsell on other services. There is nothing special about the internals and operation of a German car braking system compared to any other make.

With my 2002 Boxster letting the brake fluid go 2.5 years was long enough the fluid to become degraded enough to affect the hydraulic clutch's action which in turn affected shifting.

My 2018 MINI JCW at less than 2 years in service needed a brake fluid flush/bleed to address an overly intrusive stability control system which had worn out the *rear* brake pads.

Then my 2020 M-B Metris van at less than 2 years needed a brake fluid service to address erratic braking action. This was not the ABS acting up.

In the case of the 2018 MINI JCW and the 2020 M-B cargo van both vehicles had been on the dealer's lot and spent considerable time outside. The JCW I bought in Sacramento CA. The M-B I bought in Bentonville AR.

The Boxster and JCW MINI I drove in the east bay and there the climate was mild. But both vehicles were while parked in covered carports they were exposed to the changes in temperature.

The M-B cargo van I drove here in NW Arkansas. It was parked in a carport. Temperatures here went lower than where I lived and drove in CA and humidity here was much higher on average than in CA.

Misco -- which sells brake fluid testing equipment -- has this:

Many motor vehicle companies recommend changing the brake fluid every 12 months, while others recommend changing it every 24 months. The amount of moisture that brake fluid absorbs depends on a number of factors including the type of fluid, humidity, type of brake hoses, condition of the seals, and the vehicle’s mileage.

Generally, brake fluid will absorb at a rate of 1% or more moisture per year of service life. A two-year-old car will have 2-3% water in the brake fluid. 2% water reduces the boiling point of DOT3 brake fluid by approximately 135 degrees Fahrenheit (75 degrees Celsius). 2% water in DOT4 brake fluid reduces boiling point by 81 degrees Fahrenheit (45 degrees Celsius).

Most of us don't use the brakes hard enough that the lower boiling point is a real problem.

But my experience has been brake fluid has been degraded enough in one case after just 2.5 years and in 2 other cases with less than 2 years of service -- but with extended exposure to the "elements" for some time before I bought the vehicle and began driving it -- that replacement was clearly called for and proved to be the cure.

[Redd]

My track student nearly sent me into the wall once with total brake failure at the end of a long straight due to boiling fluid. When I asked him when was the last time he changed his brake fluid, his response was "Do I need to change it?"

So yeah, change your brake fluid.

[BlkGS]

Originally Posted by XutvJet

I don't think any of us are saying that. Most of us understand that vehicle fluids have a life expectancy. It also pays to test fluids and there are plenty of cheap products on the market that you can use to test brake fluid, coolant, and such. It is a dealer/shop's vested interest to sell you on all sorts of maintenance services as they make them a ton of money.

BMW's 2 year brake fluid service interval is largely based on folks driving high speeds on the autobahn where high MPH stops can result in potential brake fade issues. Us in the US typically are driving well below 50mph and on the highways, 80mph and usually more like 65mph in most places on average. Brake fade isn't much of an issue during a single panic stop from 80mph to 30mph vs one at 120mph to 40mph. The German makes are just bonkers about their brake fluid changes but if you go to an Asian or domestic shop, they hardly ever recommend a brake fluid change unless it's been years, the car really old, and/or the fluid in the reservoir brownish. Those cars use DOT 3 or 4.

Lastly, I've tested 4 y/o brake fluid on my M235 at the reservoir and at the caliper. Hardly any measurable difference and within 0.5% of a percent, thus, so much for that argument and it really opened my eyes to how needless a 2 year fluid change is on a street driven only car. Now if I was tracking any of my cars on occasion, I'd be changing the fluid probably every year for obvious reasons.

You've never been to Florida, have you? Lol. Our highways move at 80+. You can be going 80 in the middle lane and have someone blow past you going WAY over that. 50 mph is like, surface roads speeds.

That said 2 years is probably more to drive service dollars than anything else. I can also feel a difference in old brake fluid vs new in the pedal, old fluid leads to a sponge feeling pedal. My C4 is due for fluid because of this feeling (to be fair, this fluid is several years old and desperately needs replaced).

All that said, I'm surprised more people don't run Castrol SRF. It's DOT4 and not hygroscopic.

[BlkGS]

Originally Posted by Redd

My track student nearly sent me into the wall once with total brake failure at the end of a long straight due to boiling fluid. When I asked him when was the last time he changed his brake fluid, his response was "Do I need to change it?"

So yeah, change your brake fluid.

Been there, done that, was lucky enough to stay calm and avoid the traffic and spin into the dirt.

That was the last time I trusted that someone had done upkeep they said they did. I had just bought this car knowing I was taking it to a track day in a week or so. Knew it needed some suspension work, so I spent the whole week doing that. Before I drove off with the car I asked the previous owner if the brake fluid was fresh. He said "yeah I just did it", he had not, the fluid was old and easily boiled. Now I replace brake fluid before every track even, and at every brake pad/rotor type service.