bildr
Posts: 1
Joined: Wed Jan 16, 2019 11:28 am

Brake fluid replacement

Thu Jun 13, 2019 7:48 am

Can anyone tell me why BMW says that my brake fluid should be replaced at the 4 year service. It only has 30,000 miles on it and I hardly ever even use the brakes. Seems like a waste.

TOEd
Posts: 97
Joined: Sat Mar 05, 2016 5:15 am
Location: Toronto Ontario Canada

Re: Brake fluid replacement

Thu Jun 13, 2019 9:09 am

In my experience the 2 year interval recommended by the dealer for our ev is a waste.
In the old days we never changed brake fluid. It was only flushed during a brake job. To this day many older guys refuse to do this preventive work.
https://bottomlineinc.com/life/car-main ... d-of-flush
Corrosion inhibitors in brake fluid must be pretty good these days. Voltmeter test is based on principle of galvanic corrosion that occurs in presence of moisture, a reading of less than 0.3 volts is ok when measured between negative battery terminal and the brake fluid reservoir. This voltage is temperature dependant so I would err on the side of caution in cold weather. I tried this on my 2013 Honda CRV (reading was 0.020 volts) and my 2015 BMW i3 (reading was 0.012 volts). Low values indicate that brake fluid is very dry. Brake lines on Honda were bled during brake job that I performed last summer. Dealer bled brakes (for free) last Fall.

vreihen
Posts: 87
Joined: Tue Apr 30, 2019 4:55 pm
Location: Orange County, NY (FN21vm)

Re: Brake fluid replacement

Thu Jun 13, 2019 9:11 am

Brake fluid is hygroscopic, which means that it absorbs water from the air. Estimates are that it absorbs 2-3% water per year. Water doesn't compress as well as brake fluid, and also causes corrosion in the system. If it was better or the same as brake fluid, trust me the manufacturers would be filling brake reservoirs with water to save a penny! It is also why you should never use the contents of a previously-opened bottle of brake fluid to top off your car.

In a normal car, the brakes generate heat with every stop. That heat helps to boil some of the water out of the brake fluid, since water has a lower boiling temperature than brake fluid. Manufacturers who specify fluid change intervals on ICE cars frequently use 3 years, regardless of mileage.

On an EV, regenerative braking limits the waste heat that boils the water out of the fluid when you're not using the brakes hard. The BMW-specified two year change interval regardless of mileage sounds reasonable to me!

Given that the two year brake fluid change is the only scheduled maintenance on the BEV model that I know of, it isn't exactly going to break the bank to pay for this service every two years. This and the scheduled two-year dealer inspection cost me a smidge over $300 last month when I paid out-of-pocket for my new-to-me 2015 i3's scheduled fluid change. I'm afraid to look up the cost of a replacement ABS pump, to see how much it would cost me if it was damaged due to corrosion from water in the fluid.....
2015 BMW i3 BEV, Giga World, Tech and Driving Assistant packages, 15K miles

jadnashuanh
Posts: 4488
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Brake fluid replacement

Thu Jun 13, 2019 5:10 pm

Brake components are expensive especially the ABS controller/pump. Moisture can cause things to corrode from inside. Preventive service is a good idea, and a staple of German tech. As noted, the moisture absorption is constant, and moisture is the culprit for that corrosion. Best to avoid that by replacing it on a timely schedule. It has no relationship with the miles driven, it's time. The boiling point also drops as moisture is introduced. This could be a big issue if you live in hilly or mountainous areas. Ignore at your own risk. There are tools you can use that can measure the moisture content, and you could use that to determine if it's time to replace the stuff. There are also test strips that can measure other problems.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

Srivenkat
Posts: 78
Joined: Sat Apr 21, 2018 11:27 am

Re: Brake fluid replacement

Thu Jun 13, 2019 6:58 pm

I am curious about the difference between brake fluid in an unopended can and the same in a sealed brake system. Specifically I would be interested to know if there are points of entry for moisture to enter the brake system. Or could moisture creep into an unopened can as well over time? I heard some cars have a rubber lid that can expand into the tank as the brake pads wear to prevent moisture from entering the system from the air that would be sucked in otherwise. Does the BMW brake reservoir use such a rubber lid? Thanks in advance.

jadnashuanh
Posts: 4488
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Brake fluid replacement

Thu Jun 13, 2019 7:58 pm

While lots of the brake system is metal, there are some rubber components (seals) and some sections of flexible rubber hoses. Any rubber seal or hose can let some moisture into the system. The brake fluid REALLY likes water...it will suck it through the rubber. That's one reason why they always tell you to only add brake fluid from a new, unopened can...once you open it, you'll likely never reseal it well enough to prevent that action from happening.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

i3Alan
Posts: 253
Joined: Wed Jul 09, 2014 4:08 pm

Re: Brake fluid replacement

Fri Jun 14, 2019 9:33 am

vreihen wrote:
Thu Jun 13, 2019 9:11 am
Brake fluid is hygroscopic, which means that it absorbs water from the air. Estimates are that it absorbs 2-3% water per year.
Does it make any difference if the year-round climate has an average humidity well over 50% (most of the eastern US) as compared to the Sonoran desert surrounding Phoenix where the average humidity is 10-20%? That is, if a 2 year flush is enough for New Orleans, wouldn't a 4 year flush interval be more than enough for Phoenix?

jadnashuanh
Posts: 4488
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Brake fluid replacement

Fri Jun 14, 2019 10:41 am

i3Alan wrote:
Fri Jun 14, 2019 9:33 am
vreihen wrote:
Thu Jun 13, 2019 9:11 am
Brake fluid is hygroscopic, which means that it absorbs water from the air. Estimates are that it absorbs 2-3% water per year.
Does it make any difference if the year-round climate has an average humidity well over 50% (most of the eastern US) as compared to the Sonoran desert surrounding Phoenix where the average humidity is 10-20%? That is, if a 2 year flush is enough for New Orleans, wouldn't a 4 year flush interval be more than enough for Phoenix?
Maybe, but is it worth the savings? Cost is less than $100 done at a BMW dealership, and probably less elsewhere. I think you'll find that the cap on the reservoir tends to let air in, but not out easily, so heating up the fluid to drive off moisture, at least in a non-track situation, probably isn't a big factor...once in, it stays in. It needs to let air in to compensate for the level dropping as the brake pads wear...not very fast in the case of the i3!

AT least in the USA, that service is covered at least once during the free maintenance period.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

Srivenkat
Posts: 78
Joined: Sat Apr 21, 2018 11:27 am

Re: Brake fluid replacement

Fri Jun 14, 2019 1:55 pm

Especially with the rest of the vehicle being rustproof (CFRP, etc.) and long-lasting, I would be inclined to maintain the brake lines well. I would be looking to get twice the life of a normal (metal) car out of this, with a battery and strut/spring upgrade at some point, as needed.

PBNB
Posts: 121
Joined: Sat Jul 21, 2018 11:28 am
Location: Vancouver, BC

Re: Brake fluid replacement

Fri Jun 14, 2019 3:54 pm

Mine is due this month as well.

I bought an extended warranty so I guess I had better make sure it gets done to avoid any issues.
2015 i3 REX

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