Everything You Always Wanted to Know About Brake Fluid
(And are probably still afraid to ask)
by James Walker
Without a doubt, changing brake fluid is at the absolute bottom of most owners’ lists of “fun things to do with their trucks.” Unfortunately, this mystery fluid is also one of the most vital components of all of your truck’s safety systems, yet it can be neglected for years and years at a time. Heck, there are some people who would not change their fluid for the life of their truck without even batting an eye.
If you are one of these people, don’t worry—help has arrived. Here is everything you will ever need to know about the very lifeblood of your truck’s braking system. If you are not itching to run to the garage by the time you are done reading, you might want to check your pulse.
Okay, maybe that comment about checking your pulse was an overstatement, but you get the idea…
What exactly does brake fluid do?
Brake fluid’s sole function is to transmit the pressure generated in the brake master cylinder to the four wheel brake assemblies. At the wheel ends, this pressurized fluid energy is ultimately translated into brake torque through another set of piston-based devices—calipers in the case of disc brakes, or wheel cylinders in the case of drum brakes.
Now, while that may sound simple and straightforward, one needs to remember the various demands placed on the fluid itself. Just to name a few:
- The fluid must not solidify (freeze)
- The fluid must not vaporize (boil or fade)
- The fluid must be compatible with rubber seals
- The fluid must not be excessively compressible
Because real-world temperatures in North America routinely fall below -40F (and even colder in extreme locations), we have an idea of just how low the freezing point must be if we also want to add in a safety factor. Realistically, brake fluid itself has such a low freezing point that this limit is rarely, if ever, a concern.
High temperature performance, however, is a completely different animal. Not only does the fluid need to be robust to high ambient temperatures (130F and higher in Death Valley, for example), but one also needs to consider that the brakes themselves generate a significant amount of heat during operation. (Quite frankly, Death Valley looks like an icebox in comparison.)
During heavy driving, hauling, or towing, it is not uncommon to see brake pad and rotor temperatures in excess of 800F. While not all of that heat is inflicted on the caliper, the brake fluid inside can easily experience temperatures in the 300F-400F range with prolonged exposure. Yikes!
Fortunately for us, brake fluid manufacturers have found a way to formulate brake fluids that meet these extreme operating conditions. Unfortunately, these very same fluids have a skeleton in the closet…
Water: the silent, wet enemy
DOT 3, DOT 4, and DOT 5.1 brake fluids (more on these terms later) are based on glycol ether-based stocks, and as a result are hydroscopic in nature. In plain English, they absorb water like there is no tomorrow. (Conspicuous by its absence from the list above is DOT 5 fluid. Unlike DOT 3, 4, and 5.1, DOT 5 fluid does not naturally absorb any water whatsoever. More on this later.)
When brand new, common brake fluids can have boiling points of well over 400F without really even breaking a sweat. However, if even a minute amount of water is absorbed into the fluid (less than 5% of the brake fluid volume, for example), the boiling point can plummet to less than half of the value when new. For this reason, brake fluids have two advertised boiling points—dry (new) and wet (used).
This is also the reason that brake fluid comes in sealed containers. Once the seal is broken, the irreversible process of water absorption begins. Note also that once you open a new container of fluid you should either use the entire contents or discard the remaining portion. Brake fluid left on the shelf for a few years will degrade rapidly in boiling point performance. Figure 1 shows a typical seal.
Why the heck do we use brake fluids that absorb water in the first place?
Believe it or not, one of a brake fluid’s most vital characteristics is its ability to absorb water. Yes, you read that correctly—brake fluids absorb water by design and that is really a good thing.
Whether we like it or not, water is everywhere and finds its way into everything. That’s just the nature of the beast. Given enough time, even a brand-new sealed brake system will eventually absorb water.
The magic of diffusion allows moisture in the air to permeate microscopic pores in the rubber brake hoses, the nylon master cylinder reservoir, and the various rubber seals in the hydraulic system. Sadly, there is nothing we can do about it, and if left unchecked the water would sit in our brake system and rot it away from the inside out. If you never change your brake fluid, this is exactly what will happen.
Hence the need for brake fluid to absorb this unwanted house guest. Because brake fluid absorbs water into solution, the local concentration levels are typically low enough that corrosion is slowed dramatically. As an added benefit, when exposed to low temperatures the solution state prevents the water from pooling and freezing on its own. While water in brake fluid will certainly increase the solution viscosity at low temperatures, this is much more desirable than having little chunks of ice plugging up the system!
So, the next time you are bleeding your brakes to remove the water-contaminated fluid, don’t curse at the automotive gods too loudly. After all, the fluid was only doing its job.
So, what exactly is the DOT rating telling us? More importantly, what is the DOT rating NOT telling us? A quick look at FMVSS116 (the U.S. Government’s regulation for brake fluids, composed of no less than twenty-two pages of brake fluid minutiae) will tell us all we need to know.
DOT 3 Fluid
DOT 3 fluids (such as shown in Figure 2) are usually glycol ether- based, but that is not because they are required to be. In fact, FMVSS116 makes no mention whatsoever about the chemical compounding of brake fluids—it simply dictates the fluid physical properties. However, the brake fluid industry has by consensus decreed that glycol-ether fluids are the most economical way to meet the requirements. So, there you are.
These glycol-ether fluids are typically a by-product of the process used to make certain paints and varnishes. By definition, DOT 3 fluids must have a minimum dry boiling point (measured with 0% water by volume) of 401F and a minimum wet boiling point (measured with 3.7% water by volume) of 284F. That’s really about all the specification says as far as the average consumer is concerned.
DOT 4 Fluid
DOT 4 fluids are also glycol-ether based, but have a measure of borate esters thrown in for increased immunity to water absorption. Because of this chemistry, the DOT 4 fluid will have a more stable boiling point during the early portion of its life, but ironically, once the fluid does actually begin to absorb water, its boiling point will typically fall off more rapidly than a typical DOT 3. By FMVSS116 standards, DOT 4 fluids must have a minimum dry boiling point of 446F and a minimum wet boiling point of 311F.
Is DOT 4 Better?
Does this make DOT 4 fluids better than DOT 3 fluids? Not always. Remember, the boiling points listed are minimums, and there are DOT 3 fluids out there with higher boiling points than some DOT 4 fluids. The real differentiating factor should be that if you run a DOT 4 fluid you really should change the fluid more often than if you use a DOT 3, if for no other reason than the rapid fall off in boiling point with time.
DOT 5: A Different Animal
On their own, silicone-based DOT 5 fluids are entirely different animals than DOT 3 and 4 fluids. Their high boiling points—509F dry and 356F wet—make them appear at first glance like just the ticket for severe-duty applications. In addition, they also tend to have much, much lower viscosities, which improves cold weather performance dramatically.
Why not just pour it in and go? One side effect of this chemistry is that there is more “room” for air to fit in between the individual molecules of brake fluid than in DOT 3 or 4 fluids. Note that we are not talking about big bubbles of air visible to the naked eye, but rather microscopic amounts of air which are finely dispersed (entrained) in the brake fluid matrix.
Now, all fluids have a certain amount of compressibility to start with, but adding even the smallest amount of air into the solution can dramatically increase the amount of elasticity in the system. In the case of silicone-based fluids, air is quite happy to take up residence between the brake fluid molecules, and, as a result, the fluid compressibility goes up. This is felt at your foot like stepping on a big spring. As you can imagine, more air = more spring. For this reason, silicone-based DOT 5 fluids are typically not favored in applications where high brake line pressures are present or when firm brake pedal feel is a critical design target.
Finally, because of the unique chemistry of the DOT 5 fluids, they cannot be mixed with DOT 3, 4, or 5.1 fluids. Think “oil and water.” Because it is relatively impossible to completely purge the system of old fluid when doing a fluid change, pockets of the silicone-based fluid will always remain isolated from the ether-based fluid. This can result in areas of localized water content, or areas of varying boiling points.
Because most modern vehicles come from the factory with DOT 3 or 4 fluids, it is a safe assumption that you should not even consider putting DOT 5 in your truck. In fact, the only production vehicles sold in the US that come from the factory today with DOT 5 fluids are Harley-Davidson motorcycles. Why? Because DOT 3, 4, and 5.1 fluids will mar the fancy paint on these machines if spilled; DOT 5 will not.
DOT 5.1 Fluid
Historically, DOT 5-level performance (specifically boiling points and viscosity) could only be achieved with silicone-based fluids. However, modern compounding has created glycol ether-based fluids which now meet DOT 5 bogeys in these key areas. Consequently, the DOT 5.1 moniker was created to differentiate between these two very different chemistries, which both meet DOT 5 performance requirements.
In so many words, DOT 5.1 fluids (see Figure 3) are simply DOT 4-type fluids which meet DOT 5 performance requirements. Because of this, they typically can be mixed with DOT 3 or 4 fluids without concern. In some circles, they are even referred to as ‘DOT 4 Plus’ or ‘Super DOT 4’ fluids because they are more similar to a conventional DOT 4 fluid by chemistry than they are to a conventional DOT 5 fluid.
While it may not be obvious, the big advantage of the DOT 5.1 fluids is that they contain all of the nifty water-absorbing characteristics of the DOT 3 and 4 fluids while simultaneously providing for very high boiling points and relatively stable viscosity over a wide range of temperatures. The best of all worlds, you could say.
So, what is the downside of the DOT 5.1 fluids? Like most things in life, the good stuff isn’t cheap. DOT 5.1 fluids typically cost three to four times as much to manufacture as conventional DOT 4 fluid. There’s always a catch, but you get what you pay for.
In summary, the chart in Figure 4 does a good job of comparing the four categories of brake fluid and their respective characteristics.
Picking the Right Fluid
Ultimately, there is no magic here. However, be forewarned that if you are working your truck hard, there are NO fluids which allow you to run indefinitely without periodic bleeding or replacement. The best that a fluid can do for you is provide stable, consistent performance during use; but because all fluids will absorb water over time, all fluids must be bled at some point. It’s that simple.
As a rule, feel free to experiment with DOT 3, 4, and 5.1 fluids to find the right brand for your application, but steer clear of DOT 5. Remember, the DOT 5 silicone-based fluids are not miscible (a fancy way of saying compatible) with the ether-based fluids. Leave the DOT 5 stuff for the two-wheel crowd!
The chart in Figure 4 does a good job of comparing the four categories of brake fluid and their respective characteristics. Use it to find the best DOT 3, 4, or 5.1 fluid that fits your budget and is readily available to you. If your fluid never boils, you’ve arrived—there’s your “right” fluid. However, if fluid fade persists, you may have to bite the bullet and pay up the ladder for the next best thing.
James also wrote the book High Performance Brake Systems that was published by Car Tech back in 2007.
James Walker’s book, High Performance Brake Systems.