does flushing brake fluid help the brake pedal feel firmer & stop faster? - Page 2 - Mercedes-Benz Forum

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post #11 of 14 (permalink) Old 07-09-2007, 09:37 AM
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Originally Posted by johnhef
I never noticed a difference in all the new cars I work on, but I just changed the fluid in a 240D that had been sitting for at least two years and who knows how long it had last been changed before that. The brake fluid was black and it had a extremely soft pedal and I was suspecting frozen calipers but after a brake flush, what a world of difference. It may still have a stuck piston or two but it feels at least 70% better, the pedal is firm again.
But how could you know the improvement was not from bleeding? As long as it's got some liquid and no air, it should not feel soft.

Perhaps if it has produced foam, like water in engine oil, then it would be impossible to get the air away with bleeding (if not putting new fluid in, which would be normal with bleeding).

In any case, fluid change should have been done a lot earlier already because of the risk of corrosion.
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post #12 of 14 (permalink) Old 07-09-2007, 03:37 PM
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true. it may have had air in it. either way don't let it get that bad

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post #13 of 14 (permalink) Old 07-09-2007, 04:59 PM
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Originally Posted by ProV1x
as old brake fluid degrades slowly over time, do the brakes start to perform worse? is there a noticeable drop in pedal firmness and how soon the brakes grab?

Yes, yes, and yes - however, brake feel is a relative term. That and it depends on how the car is driven. In a normal stop-and-go environment, it's tough to tell whether a system is filled with fresh fluid or 5 year old sludge. A few laps on track day will expose old fluid. Brakes get hot. They get hot enough to boil the fluid in the calipers. When brake fluid boils, it changes from a liquid to a gas. Gas is compressible. That compressible gas is what causes the long pedal travel and the spongy "feel". If this is ignored for a few laps, the brakes can fail completely. When this happens, the driver is typically elected to the "entertainment committee".

I rotate between ATE Blue, and Gold every other year. It's a DOT4 fluid and according to the container in my garage, it's rated at 536˚F/392˚F. Those two numbers are the dry and wet boiling points. Brake fluid is hygroscopic, which means it absorbs moisture. As it absorbs moisture, the boiling point lowers, which reduces it's performance. After a few years, that firm pedal that initially required 536˚ before the fluid boils, now starts boiling at 70% of what it took when the fluid was new. On track days, this lower boiling point forces one to start braking a lot earlier. The reason I rotate between those two colors is because when I flush the system, I can see that I'm done at one corner because the color changes.
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post #14 of 14 (permalink) Old 07-09-2007, 10:58 PM
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The technical facts from MarcusF make it easier to understand the issue. Thanks.
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