Without a doubt, brake fluid is near the top of most enthusiasts’ lists of boring brake subjects. It may even rival the ashtray and cup holder for the all-time most boring vehicle part ever. Yet in spite of its low score on the “cool-o-meter,” brake fluid is one of the most vital components to your vehicle’s brake system performance, and ultimately to its overall safety.
How then can it be neglected for years and years and years at a time? This may not surprise you, but some people don’t change their brake fluid for the entire life of their vehicle and don’t even think twice about it.
But don’t worry—help has arrived. You’re about to learn everything you ever need to know about the very lifeblood of your vehicle’s brake system. If you’re not itching to run to the garage with a bleeder bottle by the time you’re done reading this chapter, you might want to check your pulse.
Time, temperature, and moisture are the three primary enemies of your vehicle’s brake fluid. The only visual indication of degraded brake fluid performance is a change in fluid color over time from light yellow (the beaker on the right) to a dark, muddy brown (the beaker on the left). (Randall Shafer)
Brake Fluid 101
Brake fluid’s functional responsibility is to transmit the force from the master cylinder piston or pistons to the calipers at the four corners of the vehicle. It does this by allowing itself to be pressurized. Based on the hydraulic gain of the brake system, there may be an increase in the force delivered to the calipers, but the brake fluid and distribution system fundamentally do not provide any gain on their own. This relationship was covered in Chapter 3, but can be summarized by the following equation:
Caliper input pressure (psi) =
Master cylinder pressure (psi)
The brake fluid found in the master cylinder reservoir is not pressurized, even during brake application. This fluid is drawn into the hydraulic circuit only as needed due to long-term changes in brake system volumetric consumption, such as when brake pads are worn down to their backing plates. (Randall Shafer/Delphi Corporation)
While that may sound simple, there are several other demands placed on the brake fluid that could prevent it from performing its intended function. For this reason, the National Highway Traffic Safety Administration, or NHTSA, an agency of the Department of Transportation, or DOT, has established a Federal Motor Vehicle Safety Standard, or FMVSS, dedicated to brake fluid performance.
In FMVSS116, NHTSA has identified no less than 14 properties of brake fluid worthy of government regulation. By law, if the fluid cannot pass one or more of the 14 requirements the product cannot claim compliance to DOT standards.
If all of FMVSS116 were to be duplicated here in this book, over 22 pages of text would be required. The sidebar provides a more detailed list of these items, but for now it makes the most sense to summarize four of the key requirement categories from the perspective of the typical automotive enthusiast.
The Fluid Should Not Solidify
As stated a few paragraphs back, brake fluid is able to transmit force across a distance because it able to be pressurized. While this may be obvious, the pressurization of brake fluid is much easier to accomplish when it’s in its natural fluid state than when it is frozen solid in the brake lines and hoses.
Therefore, a critical requirement for brake fluid is that it must maintain its fluid state even in the presence of extremely low ambient temperatures. Because ambient temperatures routinely fall well below 32 degrees F, this immediately eliminates water from the list of potential brake fluids.
One of brake fluid’s most important characteristics is its ability to maintain a fluid state even in the presence of extremely cold ambient temperatures. For this reason alone, water would make a very poor brake fluid in climates where temperatures drop below freezing! (Randall Shafer)
The Fluid Should Not Vaporize
As stated a few paragraphs back, brake fluid is able to transmit force across a distance because it’s able to be pressurized. While this may be just as obvious as it was moments ago, the pressurization of brake fluid is much easier to accomplish when it’s in its natural fluid state than after it has boiled into gaseous form in the brake lines and hoses.
Brake fluid fade occurs if the brake fluid vaporizes during use (note that this is quite different from brake pad fade, which is covered in Chapter 9) and is characterized by a brake pedal that falls nearly to the floor of the vehicle when pressed by the driver. This increase in pedal travel is accompanied by a partial or complete loss of deceleration capability, which results in extended stopping distances. This condition tends to make drivers a bit uncomfortable, to say the least.
Brake fluid fade occurs when the brake fluid changes in state from a liquid to a gas. Unfortunately for the driver, brake fluid in vapor form is much more compressible than it is in liquid form, resulting in little, if any, braking effectiveness. Under these circumstances, the best one can hope for is a gravel trap to stop the car. (Wayne Flynn/pdxsports.com)
Therefore, a critical requirement for brake fluid is that it must maintain its fluid state even in the presence of extremely high ambient temperatures. However, there is a large amount of heat generated by the brake system itself. In fact, the heat coming from the brake system can greatly overshadow any effects of ambient heat. Remember from Chapter 1 that rotor temperatures of hundreds of degrees Fahrenheit are not all that uncommon, even in everyday driving.
For this reason, the boiling point of brake fluid is one of its important physical attributes. In fact, to the racer it may be the single most critical performance criteria.
Of paramount important to the racer, brake fluid must also maintain its fluid state in the presence of extremely high temperatures. For reliable performance on the track, dry boiling points well over 500 degrees F are often required. (Wayne Flynn/pdxsports.com)
The Fluid Should Not Attack Seals
Before you get the impression that I’m referring to the protection of an endangered species, this requirement simply implies that brake fluid should not adversely affect the performance or longevity of the hydraulic seals in the master cylinder, proportioning valves, and calipers. This requirement not only applies to chemical compatibility, but also to any lubricating properties that the fluid may need to possess.
(Author’s note: no seals were harmed in the writing of this book.)
The Fluid Should Not Be
When Newton (yes, the same Newton from Chapter 2) was not preoccupied defining the laws of motion, he took some time to study fluids and discovered that all of them—without exception—were compressible to some degree. In other words, when they are pressurized they decrease in volume. For this reason, when brake fluid is pressurized it shrinks in reaction.
While this does not pose a performance concern with the fluid, it does result in extra brake pedal travel. For this reason, selecting a brake fluid with low compressibility can result in significant improvements in brake pedal feel.
Brake fluid’s compressibility indicates how much the fluid shrinks in volume when it’s pressurized. For the best brake pedal feel, fluid with the lowest compressibility possible is desired. This is just one of the reasons that silicone-based DOT 5 fluid is not recommended in high-performance applications—its chemistry makes it much more compressible than DOT 3 and DOT 4 fluids. (Randall Shafer)
Dry Boiling Point
During competitive events, mild track use, or even spirited street driving, it’s not uncommon to see rotor temperatures well in excess of 800 degrees F. While not all of this heat is seen by the caliper body, the brake fluid behind the piston can easily reach 300 degrees F, with severe applications resulting in even higher temperatures.
Fortunately, brake fluid manufacturers have found a way to formulate brake fluids that can perform in these extreme operating conditions without even breaking a sweat. When new, most common brake fluids have dry boiling points of well over 400 degrees F. Unfortunately though, these very same fluids also have an Achilles heel.
Although the glowing front brakes on this Subaru indicate their temperature is well over 1,200 degrees F, the brake fluid does not experience quite the same level of heat during use. The temperature of the brake fluid in the caliper is probably closer to 300 to 400 degrees F. (Wayne Flynn/pdxsports.com)
Detailed FMVSS116 Requirements
In all, FMVSS116 calls out 14 different performance requirements over 22 pages for brake fluid sold in the United States. Listed here, in their official order of appearance, are those properties under NHTSA’s microscope:
1. Dry boiling point
2. Wet boiling point
3. Kinematic viscosity
4. pH value
5. Chemical stability
7. Fluidity and appearance at low temperature
9. Water tolerance
11. Resistance to oxidation
12. Effects on rubber
13. Stroking properties (lubrication capability)
14. Fluid color
While each requirement spells out specific performance criteria in gory detail, all were developed with a single objective in mind: ensuring that brake fluid maintains its ability to transmit force under a wide range of operating conditions.