Let's Talk Brake Fluid

A forum to post write-ups and how-tos for Honda Preludes.

Moderators: RedRacer, Moderators

Post Reply

How often do you change your brake fluid ?

Every Year
Every Few Years
What's Brake Fluid?
Total votes: 6

User avatar
Posts: 532
Joined: Sat Nov 19, 2005 10:38 pm
Prelude Model: 1995 Si w/mods
Location: Alabama

Let's Talk Brake Fluid

Post by RedRacer »

Brake fluid. Most people just know that it has to be checked and that it should never get low. They don't know that it should be flushed annually for most average store bought brands, nor are most people aware of the many variances and differences in brake fluids and types.

First, you need to understand wet and dry boiling points and how they impact your ability to stop safely
(unless you're one of those who never changes your brake fluid or cares...and in that case, close your browser window now)

The amount of moisture in brake fluid definitely affects its performance. The big problem is it absorbs moisture quickly. Over a relatively short period of time brake fluid will absorb moisture from the air. SAE field tests have shown that the average one year old car has 2% moisture in the fluid. A random test of vehicles in the U.S. showed an average water content of 2.6% for vehicles with an average age of 8 years. And 25% of these vehicles had water content greater than 4%. That's really amazing AND scary to me! I drive on the same roads with most of these people....and so do YOU! You may even be one of them, RIGHT NOW!

As water content in brake fluid increases over time, the boiling point decreases. Fluid with a reduced boiling point (or high water content) can create vapor by boiling in the caliper, or wheel cylinder. The result is sudden brake failure. And water in the brake fluid can contribute to corrosion of parts such as steel pistons and ABS modulators.

The end result is, even though DOT 3 fluid is rated at greater than 401' F, in the typical 3 to 4 year old car with 3 to 4% moisture content, it could boil at temps under 300' F! If you have more than 4% moisture content in your brake fluid, well, you may as well be running straight water in the lines! YIKES! Can you say CRAAASH?!

Let's look at the DOT ratings. The table below shows the MINIMUM wet and dry boiling points for DOT 2, 3, 4, 5 and 5.1 brake fluid in degrees Fahrenheit.

DOT 2 Dry Boiling point **
DOT 2 Wet boiling point **

DOT 3 Dry Boiling point 401
DOT 3 Wet Boiling Point 284

DOT 4 Dry Boiling Point 446
DOT 4 Wet Boiling Point 311

DOT 5 Dry Boiling Point 500
DOT 5 Wet Boiling Point 356

DOT 5.1 Dry Boiling Point 270
DOT 5.1 Wet Boiling Point 190

The **DOT 2 spec is for drum brakes and is obsolete. If you have any DOT 2 in your garage, throw it away! DOT 5 is for silicone brake fluid. Silicone brake fluid (DOT 5) should be avoided because it is not compatible with regular brake fluid, it is hard to pour without introducing bubbles and thus results in soft pedal feel, and moisture still gets into your system and will pool in low areas like your calipers and encourage rapid corrosion. STAY AWAY!

DOT 5.1 is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based). Polyethylene glycol fluids are hygroscopic and will absorb water from the atmosphere, which is necessary to prevent sheer and undiluted water in the braking system, which is very corrosive. *As of 2006 most cars produced in the U.S. use DOT 4 brake fluid.

That leaves DOT 3 and DOT 4 fluids. These fluids are compatible with each other and may be interchanged or mixed with no ill effects.

Let's look at some popular brake fluid brands/types and their boiling points: * boiling temps are Fahrenheit

Castrol LMA DOT 3 and 4
Dry - 446 Wet - 311

Ford Heavy Duty DOT 3
Dry- 550 Wet - 290

ATE Super Blue Racing
Dry - 536 Wet - 392

Dry - 536 Wet - 392

Motul Racing 600
Dry - 585 Wet - 421

Castrol SRF
Dry - 590 Wet - 518

Performance Friction
Dry - 550 Wet - 284

Castrol LMA is very good at rejecting moisture and may be kept in your brake system for a couple years. The LMA stands for Low Moisture Activity. This is the minimum quality stuff that I would use in my Grandma's Impala SS. It comes in plastic containers which do not have a long shelf life. Don't buy lots of this stuff at a time because moisture can make its way through the plastic containers. About 8 or 9 dollars a bottle.

Ford Heavy Duty DOT 3 is VERY inexpensive and is popular among racers because of its excellent dry boiling point. BUT, it does absorb moisture quickly, but the racers don't care since they change their fluid frequently. Comes in metal cans so it can be stored. I would not use this say, in my grandma's Impala SS. The car sits more than it is driven, moisture would be rampant in the system. Keep this for the racers only.

ATE Super Blue Racing and ATE TYP 200 are the same brake fluid in two different colors (blue and amber, respectively). Big advantage in using these two fluids are the colors. You can easily alternate between the two because they have the same specs, and the colors let you know when you have successfully replaced the old fluid with the new (old blue, new amber, vice versa) BMW factory recommends this brake fluid for their street cars because it, like Castrol LMA, absorbs moisture very slowly. The big advantage over LMA is that ATE has a better wet boiling point. You can put this stuff in your car and forget about it for a long time.(3 years) An excellent choice for a weekend track car which also sees regular street duty. Comes in metal cans. This is what I use in my cars, trucks and motorcycle; Prelude, G35, Titan, Tahoe and Ducati 996. Works great in all and gives very nice, firm pedals.

Motul Racing 600 is a very exotic and expensive synthetic fluid with high wet and dry boiling points. Too expensive for the street and requires frequent changing due to its hygroscopic nature. Sold in plastic bottles. It is not suitable for street use because it absorbs moisture quickly.

Castrol SRF is a hyper-exotic and hyper-expensive brake fluid that is generally used by people with lots of money who run Porsches at track events. I've seen prices of $78 a liter for this stuff. Sold in metal cans. I can't afford that!

Performance Friction High Performance DOT 3 has a good dry boiling point, but a crummy wet boiling point. It comes in metal cans which is good for shelf life and sells for about $8 dollars for a 16 ounce container. If you are even considering this fluid, I would go with the cheaper Ford Heavy Duty DOT 3. With either of them, you need to change them frequently due to the poor wet boiling point.

Moral of this story is: Flush your brake fluid every year or so. But only if you would like it to work well scarcely an inch away from those toasty warm 500 degree rotors during a couple of hard or high speed stops! Or would you rather have the squishy pedal of death?

Last edited by RedRacer on Mon Feb 04, 2008 7:25 am, edited 4 times in total.



User avatar
Posts: 2196
Joined: Fri Nov 11, 2005 2:55 am
Prelude Model: 1991 SE-SR
Location: Alberta, Canada

Post by spiffyguido »

That's a killa writeup, Josh. Love it.

This is really important stuff. Having functional brakes is what saves your life. Change the fluid often!

I'd also like to point out that similar fluid can be found in our clutch lines (for those of us with hydraulic clutches), and it too should be changed frequently.

Thanks Josh!


Lude Dude
Posts: 13
Joined: Fri Dec 07, 2007 10:34 am
Location: Hinckley,uk

Post by garygadget »

Yep good writeup
Also change mine regular,as well as bleed them after a blast on track
Guilty by reason of insanity

Lude Dude
Posts: 10
Joined: Mon Apr 28, 2008 2:21 pm

Re: Let's Talk Brake Fluid

Post by CADster »

i change mine twice a year... spring and fall.

Prelude Enthusiast
Posts: 4
Joined: Thu Feb 03, 2011 8:45 pm

Re: Let's Talk Brake Fluid

Post by si96lude »

You have convinced me. I am going to get a one man brake bleeder tomorrow and see if I can find any of the ATE fluid here in Colorado Springs. I will do my part to get the word out about this.
96 si (stock for now)

User avatar
Posts: 532
Joined: Sat Nov 19, 2005 10:38 pm
Prelude Model: 1995 Si w/mods
Location: Alabama

Re: Let's Talk Brake Fluid

Post by RedRacer »

Glad that you enjoyed the write-up. And I'm glad you commented because that made me reply which made me look over the article and realize that I need to do a little updating and cleanup on it. :) You might want to read over it again. While I didn't change much, I did go over a few updates and made it a little more "fluid" (no pun intended) in how it reads. I can't believe I posted that without proofreading it better than I did!

Since ATE Blue or TYP 200 is not a product that the average Joe buys, most parts stores don't sell it. If you feel like a ride up to Denver, Dart Auto carries it, or did. I'd call them before you make the drive, just to be sure. Not that far but with gas at 4 dollars a gallon, not a trip you want to make without knowing they have what you're going after. http://www.dartauto.com They're a really good Porsche BMW service shop in Commerce City, about 6 miles northeast of downtown Denver, past the Suncor refinery.

Have fun, and good luck.

Post Reply