DOT 3 vs DOT 4 Brake Fluid

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Does anyone really know what the brake fluid does? If you are a DIYer, you most likely understand what it does and what the differences are between DOT 3 and DOT 4.

However, most vehicle owners have no idea how the braking system works. All they know is that pushing on the brake pedal will cause the vehicle to come to a stop. There is so much more to the story.

In this article, I will explain what brake fluid does, what the different ratings mean, and how DOT 3 and DOT 4 brake fluid impacts brake system performance. This information will help you to make better decisions regarding the maintenance of your vehicle’s braking system.

What Is Brake Fluid? What Does It Do?

Rather than jumping right into an explanation of the differences between DOT 3 and DOT 4 brake fluid, you need to gain a better understanding of what brake fluid does.

This information will prepare you for how different ratings of brake fluid will impact brake system performance. Most vehicles utilize a hydraulic pump to send brake fluid to each wheel caliper. The pressure you apply to the brake pedal will cause the caliper to force the brake pads to compress on the wheel rotors. The more pressure you apply, the more friction is generated, and this action will slow down and eventually stop the vehicle.

The brake fluid makes stopping possible, so it is important to change it periodically because it can absorb moisture, which will degrade it to a point where the braking system could malfunction. You may be wondering why moisture in the brake fluid is harmful.

If gas is in your braking system, pressing the brake pedal will compress the gas instead of pushing the brake fluid to your brake pads. Gas in your brake fluid system will create a situation where the brake pedal goes to the floor, and you cannot stop.

How Is Brake Fluid Rated?

Brake fluid ratings are based on the fluid's boiling point.

Two types of measurements are used to rate brake fluid. The dry boiling point is for pure brake fluid, whereas the wet boiling point measures water-contaminated fluid. The Department of Transportation (DOT) defines the ratings for all brake fluid types, including DOT 3, DOT 4, DOT 5, and DOT 5.1.

By referring to the table below, the ratings for each type are identified. As you can see in the table, the composition of each type corresponds to the wet and dry boiling point. In many cases, the type of fluid used in a vehicle is determined by the potential for higher temperatures based on speed and heavier towing loads. In comparing DOT 3 vs DOT 4, the boiling point and lifespan are the critical variables.

Typically, DOT 4 fluid will need to be replaced more often as it absorbs moisture faster than DOT 3 fluid. The DOT 5 and DOT 5.1 brake fluids are not compatible with Antilock Braking Systems (ABS), so as a result, they are used less often than DOT 3 and DOT 4.

Due to the different rating characteristics of each fluid, it is important to never mix them and only use brake fluid that conforms to your vehicle manufacturer's recommendation. If unsure, check your owner's manual or bring your vehicle to a service center.

How Does DOT 3 and DOT 4 Brake Fluid Impact Brake System Performance?

By far, the most common type of brake fluid is DOT 3; however, DOT 4 is quickly gaining widespread use as more vehicles are equipped with ABS and Traction Control. When it comes right down to identifying the differences between DOT 3 and DOT 4 brake fluid, there are really only a few, as follows:

• DOT 3 brake fluid will absorb less water than DOT 4 over the same time period, so DOT 3 requires less frequent changing.
• DOT 4 has higher wet and dry boiling points, which make it safer at higher temperatures. For example, it is a good fluid to use in vehicles that tow heavy loads.
• The bottom line is that the main differences lie in each fluid's water absorption and ability to handle the heat.

You should be able to identify the correct type of fluid needed for your vehicle by consulting your owner's manual. If you do not have the manual, a quick web search will do the trick.

Conclusion

The intense heat generated between the brake pads and rotors requires that your brake fluid be able to handle the heat. Depending on the temperatures involved, you can choose to use DOT3 or DOT4.

High heat can vaporize brake fluid, causing it to become a gas, which results in a soft, spongy brake pedal performance. In racing circles, where high-speed acceleration and stopping are involved, a soft, spongy brake pedal is known as brake fade, and most drivers try to avoid it.

Another situation that will cause brake fade is pressing the brake pedal continuously over a long period of time, also called riding the brake. I ran into this situation when driving down from Pikes Peak in Colorado.

Over the several-mile descent, I was required to stop at checkpoints so that the attendant could measure the temperature of my brakes. If they were too hot, I had to let them cool down before continuing my descent. They took safety very seriously.

Your preventative maintenance should include checking the brake fluid with each oil change and plan to replace it at least every five years. Taking the time to check your brake fluid will ensure that you will stop at a moment's notice.

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