When it comes time to change your brake fluid, knowing the differences between DOT 3 and DOT 4 brake fluid will help you select the appropriate fluid for your vehicle.
This article will discuss the significant differences between the two types of brake fluid (DOT 3 vs. DOT 4) concerning the chemical composition of the fluids, the dry and wet boiling points of the two fluids, and the proper applications of each fluid.
Finally, we will explain what happens if you unintentionally mix the wrong brake fluid.
Differences Between DOT 3 and DOT 4 Brake Fluid
Most automotive manufacturers will use DOT 3 or DOT 4 brake fluid. DOT 5 and DOT 5.1 fluids are also becoming available on the auto parts store shelves, but just outside of the scope of those two fluids, the prototype DOT 5 and DOT 5.1 fluids have some significant differences.
Before getting into those differences in detail, let’s start with the biggest comparison:
#1. Boiling Point.
The first important comparison between the two fluids is their boiling point, which is the temperature at which the fluid vaporizes, and it is calculated in two ways.
Dry Boiling Point
The dry boiling point is the temperature where the brake fluid has not absorbed any moisture from the atmosphere, meaning it is brake fluid in the container it came in.
Under typical usage, DOT 3 brake fluid has a dry boiling point of about 401°F (205°C), and DOT 4 fluid has a dry boiling point of about 446°F (230°C).
Wet Boiling Point
Wet boiling point is the boiling point at which the brake fluid begins to boil after it has absorbed some moisture from the humidity in the air. Because DOT brake fluid is in a sealed reservoir, it will absorb low amounts of water over time.
Under typical usage, DOT 3 fluid has a wet boiling point of about 284°F (140°C), while DOT 4 fluid has a wet boiling point of about 311°F (155°C).
Why is this important? Brakes generate extreme amounts of heat from the friction when stopping your vehicle. When the brake fluid reaches its boiling point through that heat, it will create bubbles and pockets of vapor in the fluid.
The vapor is compressible like every gas, while the liquid in the brake system is incompressible in the brake system. When that happens, the brake system will feel soft and spongy. A dangerous condition known as brake fade.
With a higher boiling point, DOT 4 brake fluid offers much more resistance to brake fade. Hard braking with DOT 3 fluid while towing, or descending long hills, can get very close to its boiling point, while DOT 4 fluid will maintain a buffer above its boiling point of vaporization.
#2. Chemical Components.
The key chemical components between DOT 3 and DOT 4 brake fluids are glycol and glycol ether compounds.
DOT 3 brake fluid is made of triethylene glycol and triethylene glycol monoethyl ether. The glycol has a relatively low boiling point, and the glycol ether will help reduce water absorption.
DOT 4 brake fluid contains polyethylene glycol along with diethylene glycol. The polyethylene glycol has a higher boiling point, which provides resistance to brake fade at high temperatures.
Diethylene glycol will absorb more moisture than DOT 3 brake fluid, but has a higher boiling point.
The different glycol bases are the main reasons for the differences in boiling point thresholds in the two fluids.
Glycol ethers in both types of brake fluid will impact hygroscopic properties (the functions determining how quickly the fluid absorbs moisture from the air over time). Excess water in brake fluid, in combination with corrosive components, will lead to corrosion in brake system components.
The chemical description can sound complicated, but the key point is that DOT 3 and DOT 4 brake fluids are made with different formulations of glycols and glycol ethers to provide different performance in brake fluids.
DOT 4 takes the chemical advantage under demanding braking system requirements in severe duty and subjection to high heat.
#3. Recommended Applications.
DOT 3 brake fluid is suitable for: daily driving applications in most standard passenger vehicles, light trucks, and vans. Barring any specification to the contrary by the vehicle manufacturer, DOT 3 brake fluid will be more than adequate for normal braking needs.
DOT 4 fluid will be required for high-performance type vehicles with advanced braking systems. Sports cars, racing vehicles, high-performance luxury models, police vehicles, and ambulances will require the use of DOT 4 brake fluid under heavy repeated high-temperature braking without fade resistance.
DOT 4 fluid may be required for heavy-duty vehicles like commercial trucks and buses having air brakes and hydraulic brake systems, where heavy towing and hauling generate a high level of auxiliary heat, DOT 4 can be a top choice.
DOT 4 brake fluid can be used in any vehicle that is designed for DOT 3 fluid, or if the owner identifies in their owner’s manual that the vehicle employs DOT 3 brake fluid. The reverse, however, is not advisable. If a vehicle has a recommendation for DOT 4 brake fluid and DOT 3 is used, that is unsafe and reduces braking ability.
As always, use your owner’s manual, check with your manufacturer for the brake fluid recommended.
Accidentally Mixed DOT 3 and DOT 4?
It is understandable if you mistakenly mixed DOT 3 and DOT 4 brake fluid. After all, they are contained in similar bottles, and it is very easy to mistakenly grab one (or make the much simpler mistake of grabbing the wrong one when purchasing). But what should you do if this happens?
First off, don’t panic. The mixed fluid is still, for the short term, going to allow you to safely operate the brakes. However, the use of DOT 3 in a DOT 4 vehicle does diminish performance in two ways: it lowers the boiling point and makes the brake fluid more hygroscopic.
First, the boiling point. When you mix DOT 3 and DOT 4 brake fluid together, the boiling point of the mixed fluid will be somewhere between the boiling point ranges for DOT 3 and DOT 4. This potential may still provide a sufficient application, but the fade resistance will be reduced.
The second issue is the increased moisture absorption due to mixing in DOT 3 brake fluid. DOT 4 brake fluid is less hygroscopic than DOT 3 brake fluid. Mixing ANY amount of DOT 3 (or even the residual traces of DOT 3) brake fluid will increase the moisture absorption of the fluid.
If it is truly an emergency, then adding some DOT 3 brake fluid to a vehicle intended for DOT 4 brake fluid should not be a huge deal, but you should flush and replace it with DOT 4 brake fluid as soon as possible.
Pros and Cons of Each Type
DOT 3 brake fluid
Pros:
- It’s easy to find and is inexpensive.
- Meets bare minimum standards for most passenger vehicles.
- Performs satisfactorily in standard braking demands.
Cons:
- Its boiling point is lower than DOT 4.
- Two to probably three times more prone to moisture absorption over time.
- Not suited for heavy-duty or performance vehicles.
DOT 4 brake fluid
Pros:
- Has a higher boiling point, thus it can resist brake fade at high temperatures.
- It is less hygroscopic (less likely to absorb moisture).
- Fits the specs and demands of heavy-duty and high-performance vehicles.
Cons:
- More expensive than DOT 3.
- Longer lead time in getting it into a store.
- Overkill for a basic transportation car or light truck.
To summarize, DOT 3 fluid is cost-effective and does what is needed for general everyday driving, and DOT 4 provides impeccable performance when you’re using braking systems that can benefit from it.
However, the decision on what to use should be based on what the manufacturer specifies (see your owner’s guide).