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Brake Fluid Characteristics


Well-Known Member
It seems there is always an ongoing debate on what constitutes a "good" brake fluid. It's a bit like the Chevy vs. Ford feud that has raged on for decades. Unfortunately, a lot of brake fluid discussions are not based in fact and can easily confuse people who are looking for hard data, rather than popular opinion.

The average consumer only knows brake fluid by its Department of Transportation rating, but there's a whole lot more going on with brake fluid than those people imagine. For the individual wanting to educate himself, a quick read of DOT Federal Motor Carrier Safety Administration Regulation 571, Standard 116 can be interesting reading. You can see the minimum standard for equilibrium reflux boiling points, wet equilibrium boiling points, kinematic viscosities, stratifications, sedimentations, oxidation resistances, stroking properties and even required colorations.

The bottom line to all of this really boils down to boiling points and viscosities for the majority of car enthusiasts. But, as you will see, brake fluid selection is another case of free lunches being non-existent. And the container that fluid is sold and stored in can have tremendous effect on how well the fluid is going to work, when you break the seal.

DOT 2 fluid is a thing of the past. The standard for DOT 2 fluid were established for drum brake applications, which had very little of the heat transfer problems associated with disc brake systems.

DOT 3 fluid is a glycol ether fluid, which is simply based on economic considerations. Glycol ether will meet and exceed all the performance standards at a better price than any other fluid. DOT 3 fluid must have a dry boiling point (ERBP) of 401° F (based on 0% water by volume) and a wet ERBP of 284° F (at 3.7% water by volume). It's kinematic viscosity (rated at square millimeters per second) must be no less than 1.5 mm2 /s at 212° F, and no higher than 1,500 mm2 /s at -40° F.

Now is a good time to point out these ERBP numbers are minimum numbers and some DOT 3 fluids meet the same ERBP numbers as DOT 5 fluids. Always check the individual manufacturer's specifications.

DOT 4 fluid is also a glycol ether fluid, fortified with the addition of borate esters, to help improve the dry and wet boiling points. The minimum dry boiling point must be 446° F and the wet boiling point must be 311° F. Its minimum viscosity level is the same as DOT 3, with its maximum rating at 1,800 mm2 /s at -40° F. Which seems like a step in the right direction, but its chemical characteristics will cause its boiling points to fall off even more rapidly than DOT 3 fluid, once it begins to absorb water. And those same chemical characteristics can make the fluid look even better at ERBP, because the numbers can often reach into the 600° F range. However the fluid can barely meet the the viscosity maximum numbers, when temperatures start to drop. Because of its drastic changes in ERBP with related water absorption, DOT 4 fluid needs to be drained and replaced more often than DOT 3 fluid. In other words, DOT 4 works well in a race car that is getting minimum annual brake system overhauls, but using it for standard automotive use requires the user to maintain a strict maintenance schedule.

DOT 5 fluid is generally a silicone based brake fluid (SSBF), which by law must be made up of no less than 70% by weight of a diorgano polysiloxane. Minimum dry boiling point must 500° F and the wet boiling point must be 356° F. The minimum viscosity level is the same as DOT 3 fluid, with a maximum rating of 900 mm2 /2 at -40° F. The numbers are looking good to the DOT 5 crowd, but only until the point is raised that SSBF can achieve these numbers by introducing extremely high compressibility characteristics. And guess what? With its inherent and unacceptably high compressibility levels, DOT 5 fluid is not recommended for high performance or standard automotive use. It is often used in military applications, because of lower water absorption and corrosion rates. So much for how great and wonderful DOT 5 fluid is.

We all know a liquid cannot be compressed, which is why fluid is used in these systems. However, all fluids do boil under extreme conditions, and boiling most certainly will produce gas bubbles in the fluid. Since gas bubbles can be compressed, welcome to a soft pedal condition. Once a fluid begins to gas, the pedal gets soft and requires more stroke to achieve the same braking effects. And what fluid is known for having poor compressibility numbers? Silicone based brake fluid. Another downside to the fluid is the ease with which air bubbles are introduced to the fluid as it is poured. Since it is nearly impossible to get these bubbles back out of the system, you're aggravating the compressibility problem before you ever start bleeding the system.

But let's not overlook that SSBF has really fantastic hygroscopic qualities. Which is a good thing, since other fluids will mix with water, whilst SSBF will not. Being lighter than water, it allows water to settle into a brake system low spots. Get water in a caliper and the only way to get it out is to remove the caliper and drain it. If someone feels the need to use DOT 5 fluid, it is generally better to use it in a daily driver, than in something that is driven seasonally, since water will pool in low spots and will start corroding the components around it. And this water pooling issue really fouls up the wet boiling point ratings on SSBF. If water collects in a caliper, that water is going to boil at 212° F, no matter what the fluid manufacturer has to say. As a result, DOT 5 systems need to be flushed and bled every 12 months. Yes, DOT 3 and 4 fluids absorb water much faster than SSBF, but the water is absorbed by the glycol ether and is dispersed throughout the system, so its poorer hygroscopic numbers suddenly become a non-issue. And the hygroscopic numbers of SSBF are really meaningless when compared to the air solubility numbers. And air solubility is what gives us all the compressible gases in extreme conditions. SSBF systems should always be bled with a vacuum bleeder and the procedure should be repeated at least once or twice, 24 hours later. Properly bleed an SSBF system at sea level and then drive it to higher altitude and you will again be fighting a soft pedal, as the atmospheric pressure drops.

And please be aware SSBF is completely incompatible with all other fluids, so never add it to anything other than a system already using SSBF. It will almost immediately gel and provide terrible braking. The gel will pick up corrosion from the system and you will inevitably end up with unusual wear in the system. Inversely, if you have a system with SSBF, never, ever, never try to change to a non-SSBF fluid. You will never be able to get all the silicone out of the system and it will cause gelling when other fluids are added.

There are some non-SSBF fluids that meet the DOT 5 standards and are identified by DOT 5.1 rating.

Now, it's time to discuss the type of container fluid is sold and stored in. The best option is metal, because it is the most effective water barrier. Plastic bottles that would offer the same resistance to water absorption would end costing more than the metal can. Buy fluid fresh, in metal cans where possible, and do not store it more than 90 days.

One thing that should always be considered is the design of the components in a particular system. A vehicle that was delivered with DOT 3 fluid should not be changed over to another fluid until it is known the components in the system will all be compatible with the new fluid. If you are rebuilding a stock system with OEM replacement parts, it's best to stick with a glycol ether fluid unless you've talked to the component manufacturers to get their input on SSBF.

Now we come down to the effects glycol ether fluids have on paint. If you keep a container of water handy, to pour on possible spills, you'll suffer no paint damage. Better yet, if you learn how to pour liquids without spilling it, you'll have no problems at all. It's a bit like the joke about the two guys in the bathroom, taking a leak. The first guys finishes, zips up and heads for the door. The second guy loudly proclaims his parents taught him to wash his hands when he was finished. The first guy smiles and responds his parents taught him not to piss on his hands. SSBF won't harm paint, but neither will it perform as well in a braking system. If you're one who likes to pour brake fluid on paint, you're better off using SSBF. If you're one who wants a reliable brake fluid to get your car stopped, the glycol ether fluid is the one to use.

A good choice for nearly all applications is Castrol LMA (Low Moisture Activation). But be aware the plastic containers are poor moisture barriers, so be sure you're buying fresh product and do not store it for any length of time.

A great choice is the ATE Super Blue fluid. It exceeds all DOT 4 standards and is known to have extremely low hygroscopic qualities, which means it absorbs water very slowly.

And now you know a bit more about brake fluid, be honest and admit how many of you maintain a regular drain/flush/fill program on your braking systems. Anyone?

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