What is brake fluid, and why do you use it?
Brake fluid, a type of hydraulic fluid, transfers power within your vehicle’s brake system. It’s what ensures that when you put your foot on the brake pedal, your vehicle comes to a stop.
Like all hydraulic fluids, brake fluid is non-compressible. That means when pressure from the brake pedal is applied at one end of the brake system, the fluid in a vehicle’s brake line is forced to move through that system. This force is transferred to a vacuum powered brake boost, then all the way to the brake pads that ultimately press against a vehicle's wheels and bring it to a halt.
For this reason, brake fluid is a critical component of your vehicle. Anything that might degrade the quality of your fluid -- such as moisture absorbed from the air -- will hurt its performance and may ultimately hinder your vehicle’s braking.
What are the different types of brake fluid?
Two of the main types of brake fluid that you are likely to see are DOT 4 and DOT 5. DOT 4 is glycol-based, while DOT 5 is silicon-based. DOT 5 is distinct in that it will not absorb water and can sustain higher pressure before boiling into gas.
The higher number indicates a higher boiling point, which is a critical factor in brake fluid. Remember, hydraulic fluids work because they are non-compressible. When pressure is applied to the fluid in a closed system, that fluid is forced to move rather than shrink in volume. But that is not the case with a gas, which will often compress in response to outside pressure.
That means it’s important that brake fluid remain liquid and not boil off into a gas. It also means that you might want to choose a fluid like DOT 5, with a higher boiling point, if you’re going to be putting a lot of pressure on your braking system, which will tend to raise its temperature.
DOT 3 brake fluid
DOT 3 brake fluids are usually glycol ether based, but that is not because they are required to be. In fact, FMVSS116 doesn’t precise the chemical composition of brake fluids. It simply dictates the fluid physical properties. However, brake fluid industry has, by consensus, decreed that glycol ether fluids are the most economical way to meet the requirements.
DOT 4 brake fluids
DOT 4 brake fluids are also glycol ether based but they contain in addition borate esters in order to improve some properties including increased dry and wet boiling points. DOT 4 brake fluids have a more stable and higher boiling point during the early portion of their 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 brake fluid. By FMVSS116 standards, DOT 4 brake fluids must have a minimum dry boiling point of 230°C and a minimum wet boiling point of 155°C.
DOT 5.1 brake fluids
Historically, DOT 5-level performance (specifically boiling points and viscosity) could only be achieved with silicone-based fluids. However, modern compositions have created glycol ether-based fluids which now meet DOT 5 brake fluids requirements in these key areas. Consequently, the DOT 5.1 brake fluids level was created to differentiate these two very different chemistries which both meet DOT 5 brake fluids performance requirements.
In so many words, DOT 5.1 brake fluids are simply DOT 4-type brake fluids which meet DOT 5 brake fluids performance requirements. Because of this, they typically can be mixed with DOT 3 or DOT 4 brake fluids without concern. Sometimes, they are even referred to as ‘DOT 4 Plus’ or ‘Super DOT 4’ brake fluids because they are more similar to a conventional DOT 4 fluid by chemistry than they are to a conventional DOT 5 brake fluid. In fact, DOT 5.1 is essentially comprised of borate esters.
DOT 5 brake fluids
DOT 5 brake fluids have been developed for military applications, i.e. for vehicles that could stand in storage for years, without maintenance and had to perform immediately when required. They are superior in terms of boiling point retention and corrosion/conservation properties, since they don’t absorb water. Up to now car manufacturers have not moved to use silicone fluids for first fill regular cars for two main reasons:
- Low air solubility, which leads to a spongy brake pedal feel.
- No water solubility, moisture which enters in the system can be corrosive and can freeze at low temperatures or boil at high temperatures.
How often should you change brake fluid?
A typical rule of thumb is every one or two years, but that will depend on the recommendation of your vehicle manufacturer.
The most common threat is moisture absorbed from the air. Water in brake fluid can lower its boiling point, which will tend to reduce pressure within the braking system and hurt performance for the reasons mentioned above. In addition, moisture can begin to corrode some of the braking system’s other components.
As brake fluid ages, it tends to go from clear or light-brown to a darker color. Moisture contamination will sometimes have the same effect, so cloudy-looking brake fluid may be a sign it’s time to get it replaced. And you can have a professional check your fluid’s moisture content any time.
Can you change it yourself?
It’s generally better to have a trained mechanic change your brake fluid than to try and handle the job yourself. Old fluid must be completely drained from the system, and it’s a substance that must be handled with care. It’s also important not to mix certain grades of brake fluid. For instance, mixing a glycol-based fluid with a silicon-based fluid will cause the latter to deteriorate.