I last changed my brake fluid in my SL400 just over two years ago in May 2023. I always use a Sharpie (or similar marker) to write the date on the brake fluid reservoir so I know when to change it again. Super hot again today, so I may wait until this evening to actually do a flush. Wasn't sure if I really needed to, so I used a brake fluid tester to indicate how much water is in the fluid. From the pics, you can see that the water content is at 1%. While sitll considered OK, I figure it is time to change anyway because of the color of the fluid. (By the way, I test my $7 brake fluid tester by dipping it in some actual water and it jumps to the red zone, over 4%.)

DO NOT neglect changing your brake fluid! Repairs will be astronomical!



SL400 two year old brake fluid. 1% water content.



SL400 two year old brake fluid. Dark color.

 

I will be changing the fluid using the attached procedures from WIS.

 

I’m not familiar with Maryland weather. Is it humid there?

 

I have a similar conductivity probe that cost about $20. I found no difference in indication between 4 year old fluid and new out of the bottle fluid -- both less than 1%. Also, pure water registered full scale. I ran some tests gradually adding water to brake fluid and obtained very inconsistent readings.

I have a brake fluid refractometer on order, and will post a thread on its measurements after running tests with it.

 

I bought an inexpensive fluid tester just to check the readings of unused brake fluid that has been stored in opened containers. When I flush my brakes I seem to go thru around 3/4 of the tin. I save the unused fluid pouring it in one container.

I noticed after 4yrs the unused brake fluid still looks like new fluid in an unopened container. Since the containers seem to be air tight the readings read the same as brand new fluid.

I found a good use for the brake fluid I have been saving, when I replaced a section of corroded brake line I put that fluid in my fluid pump and hooked it up to the reservoir to make sure it doesn't go dry. Then when finished I did a flush using fresh fluid. I usually get the MB Dot4+ fluid off eBay for less than 420 which isnt bad.

I use a piece of blue painters tape to mark the fluid level in the reservoir prior to the flush. Then fill it back to this line when done. The level wont be to max since the pads and rotors arent new.







heres a new fresh container of fluid:



Unused fluid saved from opened containers:

 

It was 98°F and over 80% humidity today.

 

That humidity probably affects the moisture in your brake fluid. Probably everything else too, jeez.

Im in CA. and brake fluid doesn’t seem to get moisture contamination in 2 years. My daily driver, a beater 2006 Scion has only had one brake fluid flush, ever.