What leads you to make such a statement. What do you think happens to this fluid over time?

In my experience it works just fine in a street car and even when not changed on a reasonable schedule (about every two years) it has no downside.

 

I thought Dot 5.1 is not recommended for our car............in fact I remember seeing someone on this forum warning us to avoid those type of brake fluid.

 

DOT4 fluid is more hygroscopic than DOT3 fluid. It is recommended that it be flushed at least once a year (as opposed to the "once every three years" Honda service recommendation -- but they also recommend using only Honda DOT3 fluid).

If you track the car a lot, you will probably want to change the brake fluid more often than once per year. Twice a year is not unreasonable.

What you are paying for with this fluid is its high boiling point, and you lose that quickly as it absorbs water. If you leave it in for more than about a year, it can also absorb so much water that it starts to corrode the brake system.

I use DOT3 fluid in my clutch (because there is no advantage to DOT4 but all the same disadvantages), and DOT4 in the brakes. But I just figure on flushing the brakes twice a year.

 

You may be confusing DOT 5 fluid (silicon based fluid that is not recommended for our cars) with DOT 5.1. The extra ".1" designation makes a lot of difference in terms of compatibility with other fluids that may not be completely purged from your system.

Regarding Motul RBF 600 and other track-oriented fluids - could someone here please explain a concept that I must have all wrong. Since the Motul and other track-oriented fluids like ATE Super Blue have a high dry AND high wet boiling points, I thought that they would be desireable as a street fluid. Even if they are more hygroscopic than DOT 3 fluid, wouldn't they maintain performance better? Is it that the contaminated fluid has other adverse effects on brake components due to the higher water content, even though performance isn't seriously degraded?

TIA

 

It might be more hygroscopic but the boiling point WET is still higher than the factory fluid DRY. Where did you read that it can absorb enough moisture to hurt the system? I'd like to see literature on that.

In any case, I flush mine about every 3 months since the car has mostly all track miles on it.

 

I've read it and heard it in many places. That doesn't mean it is true, of course!

A Google reveals:
http://www.csaa.com/global/faqdetail/0,139...257C226,00.html
http://www.babcox.com/editorial/bf/bf40142.htm
http://www.carcentral.net/content/guides/H...BrakeSystem.php
etc.

None of them are particularly authoritative, but they all say basically the same thing. This is also why the silicone-based DOT5 system is recommended for collector cars -- because it won't absorb water and lead to corrosion.

 

The first one says next to nothing about DOT-4 being worse than DOT-3 in the absorption of moisture.

The second doesn't either.

The third link says "Unfortunately, DOT3 brake fluid also absorbs moisture the fastest and has the lowest boiling point. DOT4 is also fairly common but is a little more expensive. DOT4 is designed to absorb moisture slower and has a higher boiling point. "

So, care to try again?

 

No, not really.

Brake fluid should be flushed on a regular basis. I guess the question is whether DOT4 should be flushed more often tha DOT3. Many people have told me so, but that doesn't make it true. Personally, I don't care all that much, since I plan to flush it a lot more often anyway, due to tracking the car.

As for whether to use DOT4 or DOT3 on a non-tracked car (or on the clutch), I don't see any reason to use the more expensive fluid even if all the people who have assured me that DOT4 degrades faster are all wrong. (Engineering isn't determined by popular vote, so they could all be wrong.)

[edit]
But I did anyway: http://www.stoptech.com/whitepapers/brakefluid1a.htm

They unambiguously say that DOT4 fluids need to be replaced more quickly than DOT3 fluids. (Interestingly enough, they also say that DOT4 fluids are less hygroscopic than DOT3, but that the additives used break down faster when exposed to water. Whatever.)

 

Fair enough.

As you said, I too change mine much more frequently than the norm because of all the track time the car sees. In any case, I'm much happier with DOT-4 than any flavor of DOT-3 or DOT 5.1 for multiple reasons.

 

Castrol SRF - 590 dry, 518 wet
Replacement every 18 months is recommended.