Why should I flush my brake fluid every year, you ask? - PeachParts Mercedes-Benz Forum

240 Ed · #1 05-29-2002, 12:54 AM

:EDIT: Mercedes Benz Factory Service Recommendation is every two years. :EDIT:

I shall give my standard lecture on brake fluid, for those who may have mercifully missed it! Ha, Ha too late! YOU MUST READ ON...

Brake fluid is hygroscopic = Means it LOVES water.
Huh? where does it get water...?

There is a tiny hole in the master cylinder cap which allows atmospheric pressure to equalize when the level drops due to pad wear. Eventually over the year, the fluid becomes diluted with water. This water can cause rust in the system, but a worse fate than that awaits!
(wait for it)

Brake fluid has a certain boiling point. DOT 3 has a lower point than the later DOT 4. (NEVER USE DOT 3) DOT 5.1 has the highest.
DOT 5 is silicone fluid and does not absorb water, and is not included in this discussion.

Okay, here is what happens. Water dilutes the fluid, right? Right.
Unfortunately, water boils at a lower temp than brake fluid, so the boiling point of the brake fluid is now lowered.
Why is this important?
In normal use, you won't notice the difference. Brake feel stays the same. Even if you were to use straight water!
The problem comes with intense use, say coming down a mountain side with sustained braking (and pulling an enormous 80 ton yacht, say).

Your calipers heat up with the attendant friction (normal). But now, because of the compromised brake fluid, the fluid starts to boil! What happens when the water in the fluid boils?
That's correct, you get STEAM!!! Steam is a gas! gas does NOT work well at all as a hydraulic medium!!!
What happens to your brakes?
NOTHING!
It is known in the trade as "brake fade". (one of several types)
No matter how hard you press on the pedal, you won't get any more brake. You will probably start to speed up as you are going down hill. The press folks call this "...he lost his brakes"

There you have it.
Change your fluid at least once every other year (in dry climates) and if you live in Louisiana or some humid place, once a year.
Pain in the butt, huh.
Your choice, but you are now informed and can now bore other people with said lecture.

tcane · #2 05-29-2002, 01:15 AM

240 Ed:

I could not have said it better! In addition, water in brake fluid = rusting/corroding brake componets = failed brake components because of the rust/corrosion = at a minumum poor braking because componets do not move like they should (like the caliper piston not pushing on the brake pads with full force) = brake components needing R&R long before they would need to be but for not changing the brake fluid. Absorbtion of atmospheric moisture causes brake fluid to turn opaque and tells the aware owner to replace the brake fluid. I use my Mighty Vac to R&R the brake fluid every year, doing it when the humidity is low - like the M-B owners and service manuals call for.

Tom

Arthur Dalton · #3 05-29-2002, 09:23 AM

Hygro...

engatwork · #4 05-29-2002, 09:08 PM

I'm with ya'll on this. Every failure I have ever seen (outside of running the pad backing into the rotors, which I have really never seen) was caliper failure because of not changing the fluid.

gsamuelson · #5 05-30-2002, 01:15 AM

Amen!

I have only been driving newer vehicles until lately and so I never thought much about brake fluid. When I received my Unimog it had a leaky wheel cylinder. Changed the cylinder and bled the system. The crap in the main bowl was scary! The crap that came out of the lines was UGLY!!!!!!!

Just did the front brakes on my 300CD and again, UGLY!! brake fluid that looks like swamp water. I am starting to wonder if anyone ever changes their brake fluid? (present company excluded, of course).

For all those who have not changed their brake fluid, DO IT!! It is like the old Fram commercial, you either pay me now, or pay me latter. A few bucks of fluid is alot cheaper than wheel cylinders or master cylinders. My unimog wheel cylinder was $200CDN, OUCH!

rickjordan · #6 05-30-2002, 04:47 AM

Just curious as to why I have never heard of changing brake fluid until I got into German cars. My VW calls for brake fluid change every 2 years. I mentioned changing the brake fluid to co-wprker who owns a Ford, and he said he has never done that and has had no problems.

dieseldaddy · #7 05-30-2002, 08:34 AM

My wife is beginning to hate this forum.
Now I have something ELSE to do on our MBs!

Just how does one change the brake fluid? Would I suck it out the top or is there a drain?

Does anyone use DOT 5.1? Should I stick with DOT 4.

Do you have to 'bleed' anything? If so, how?

(Boy, too many question this early in the morning...)

Thanks guys!

Jim Anderson · #8 05-30-2002, 11:05 AM

If water in the brake fluid is bad why did they use a hydroscopic fluid? Why not hydraulic fluid for hydraulic brakes?

#9 05-30-2002, 11:33 AM

if used in brakes, especially, that its flashpoint is much too low -- instead of boiling and producing brake fade on the way down Pike's Peak, it might begin to burn. Besides that, viscosity stability at an extreme temperature range is very important, and only _synthetic_ hydraulic oils might have a chance here -- but would they still be compatible with the types of seal materials needed to deal with the above problems? -- cf. the silicone DOT 5 dilemma: great in British braking systems from the 50s and 60s (including my much lamented '66 Lancia Fulvia Coupe), but bad for anything else.

240 Ed · #10 05-30-2002, 07:40 PM

Jim Anderson

That is just a super question!

I dunno...

But I think blake fluid IS hydraulic fluid. But it is different than "jack oil" used in hydraulic jacks. Hmmmm.

I am sure "they" did some research and have arrived at this stuff after much testing.

Jim Anderson · #11 05-30-2002, 07:46 PM

And if water is so bad why did they use sponge fluid? Did they have square pants

DeltaDave · #12 05-30-2002, 09:41 PM

Is this string about Sponge Bob changing his brake fluid ?

MVK · #13 05-30-2002, 09:49 PM

do you folks think this will work for bleeding. It sounds good. any other suggestion on flushing/bleeding the brake system. I have to do that on my MB and my wife's Accord.

http://www.mityvac.com/techindex.html

MVK

#14 05-30-2002, 11:38 PM

Everyone has an old ear cleaner lying around. They work great for sucking brake fluid out of the reservoir. Just don't put it back in yer' ear

240 Ed · #15 05-30-2002, 11:42 PM

I gotta tell ya Bob, I am still working on this one...