Brake Fluid
Thursday, March 17th, 2005 at 10:40 pm by angrybob
This is yet another discussion similar to those already found online about the different types of brake fluid available and the “do’s & don’ts”. What I hope to add though is practical recommendations based on my brake engineering experience plus real world data from riding. I’ll skip the big words for the most part and get to the important stuff.
The one big word you need to know is hygroscopic meaning “readily absorbs water”. All DOT 3/4/5.1 fluids are glycol-based and absorb water over time. Over the years I have read that brake fluid absorbs between 2-3% water content in per year. DOT 5 is silicone-based and does not absorb water, but is more expensive and more compressible.
SIDEBAR - THESE TWO TYPES OF FLUIDS ARE NOT COMPATIBLE! It is important to note that the DOT 5 & DOT 5.1 nomenclature is not consistent between Europe and the US. Ask one of the many Aprilia owners who has followed the owner’s manual calling for DOT 3/4/5 compatible fluid and dumped in silicone-based fluid. Heh - here in the states DOT 5.1 is DOT 3/4 compatible.
Recommendation #1: Know what your bike has in it. Chances are that unless its a newer Harley, its DOT 3/4/5.1! DOT 5 is not an upgrade!
Recommendation #2: If you do mix the two, while the purist would disassemble all brake components that came in contact with the mistaken brake fluid, wipe the seals clean, and the flush / brake internals, you’ll probably be alright flushing a quart or two of brake fluid (the cheap stuff) through the system. The result of mixing the two produces a nice ‘gooh’ in your brake system that accelerates internal corrosion.
…continued from teaser here
For the rest of this discussion, we’re going to pretend that you’ve taken my advice and not gone with the silicone-based DOT 5 brake fluid regardless of the bling-bling blue/purple color. Done with silicone-based DOT 5.
The difference between the DOT brake fluids are the Dry and Wet boiling point requirements. The definition of Dry brake fluid is 0% water content or new fluid. The definition of Wet brake fluid is 3.7% water content. This water is accumulated for the most part by humidity in the air and absorption of water through external rubber parts. Every time you open your brake reservoir, you are introducing water into your brake fluid. Is this reason for concern? Nah, but just realize that leaving the cap off of your bottle of brake fluid degrades the performance relatively quick (not minutes, but hours or overnight).
As the DOT number increases, so does the boiling point. Please note that there is a good amount of overlap between the classes. Some higher grade DOT 3 brake fluids have higher boiling points than some lower grade DOT 4 fluids. This is precisely why you often see these days “DOT 3/4″ brake fluid. Anyway, here are the differences in boiling points (BP) for DOT 3/4/5.1 fluids:
DOT 3: DRY BP = 401F; WET BP = 284F
DOT 4: DRY BP = 446F; WET BP = 311F
DOT 5: DRY BP = 518F; WET BP = 375F
Using the 2% annual water content increase, you can see it takes less than 2 years to go from new fluid or DRY BP to the WET BP condition. That is why most manufacturers recommend a brake fluid flush every two years. The visual for WET fluid is that it has turned from a clear yellow to an orange or almost brown color.
Recommendation #3: Just do a brake fluid flush every two years. I know the job sucks, but its an important safety measure that has proven benefits. While you’re at it, you might as well do the clutch too. 
Finally, I want to touch on how to buy brake fluid and what brand and/or DOT level. There is no perfect answer for which brand to buy, but here’s what I do. Even as a brake engineer, I still go to any department store and buy a name brand (e.g. Gunk/Valvoline/etc.) DOT 3/4 fluid. Its readily available and its cheap. DOT 5.1 is clearly better, but its also 3-5 times as much. I have abused many sport bikes on the track and through the mountains with likely WET DOT 3 fluid and have never boiled it. Unless you are racing competitively, the DOT 3/4 blend should be very sufficient. Save your money.
I also buy only what I need plus a little to keep on the shelf. I recommend buying in pint sizes. This minimizes the wasted brake fluid because it has a shelf life…especially brake fluid in an opened (unsealed) container. Even new fluid that is still factory sealed has a shelf life of about two years. Of course you should keep an extra pint available should you have to bleed the system, but the quart or liter bottles are not wise for the amounts that motorcycle require.
Recommendation #4: For us mere mortals, DOT 3/4 fluid has plenty of performance with regard to fluid boil and should be bought in pint sizes to minimize waste and maximize the ‘freshness’.
Recommendation #5: Make yourself a reservoir for disposal brake fluid. I took a half liter soda bottle (the plastic kind with a cap), drilled a hole in the bottle cap just smaller than the size of the tubing you connect to the bleeder and fed the tuning through the hole into the bottle. Presto…you have a sealed bottle to capture spent brake fluid.
EDIT - Recommendation #6: Brake fluid is bad stuff. It eats paint among other things and should not go down the kitchen drain. Retails auto parts stores should accept used brake fluid in order for it to be disposed of properly.
So there it is, six simple recommendations for the every day rider of all types of motorcycles.
Bonus tip - if you can’t seem to get the air out of the system no matter how much fluid you pump through, try this: Place a tie-wrap around the brake lever and throttle and tighten so that moderate pressure is applied to the brakes…and kept. After twenty-four hours, cut the tie-wrap and squeeze the lever slowly a few times and you should be good. Just try it.
EDIT - place the bike on the sidestand with the bars turned to the left while it sits overnight. (hat tip - Louge)
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I read your article with interest. Here is my dilema….
I own a 2005 Screaming Eagle Electra Glide. I did not like the braking on this bike so I bought a set of PM calipers. I went to de Harley dealer to buy brake fluid, I wanted to use the VERY same stuff the bike had when it was built. The guy gave me a black bottle.
I changed the front calipers.
I removed the reservoir cover and started to bleed teh system.
I poured some of the new fluid I had bought at the Harley place……to my HORROR this thing was purple!!
At that point I decide to continue to bleed the system until purple liquid came out of both fron calipers. I rode teh bike and bled the brakes again, I rode the biek once again and bled the bkakes again. In all I used 16 oz of DOT 5 fluid to bleed teh barkes.
Now I read all details about DOT 5 and I am concerned with the integrity of my brakes.
What now? Stay with DOT 5….or go back to DOT 4?
PLease e-mail your comments soon…..on Thursday I liek to ride to Daytona for bike week and hate to have front brake failure.
Thanks,
Luis Abellon
LJABELLON@BELLSOUTH.NET