What Kind of Brake Fluid Do I Need?

You might not think much about what goes on when you press the brake pedal in your car. Behind the scenes, the brake booster amplifies the pressure from your foot and transfers it to the calipers via lines filled with hydraulic brake fluid.

Like other fluids in your car, the brake fluid needs to be changed periodically. Over time, it loses its effectiveness thanks to its hygroscopic (tendency to attract and hold water) properties. Not all brake fluid is created equal, and not all vehicles use the same type of fluid. In this article, you will learn about different brake fluid types and how to identify the variety you need to use in your car!

Brake Fluid Properties

Brake fluid needs to have a high boiling point, be incompressible, and be lubricating to work properly.

Brake Fluid is Incompressible

This is one of the most important properties of brake fluid. To effectively transfer the force of your foot on the pedal to the brakes, the fluid cannot compress. If it did, the brakes would not be as strong, and the pedal would be spongy.

Brake Fluid has a High Boiling Point

Brakes are all about friction and pressure, both of which generate heat. If the fluid in the lines boils and turns to gas, it becomes compressible, and the effectiveness will drop. This phenomenon is known as brake fade. While it is most common in racing environments (due to high speeds and heavy braking), street vehicles can experience it too. Brake fade is particularly prevalent in vehicles that tow trailers or drive up and down long, steep mountain roads.

Dry vs. Wet Boiling Point

Brake fluid boiling points have two ratings. The dry boiling point is the temperature at which the fluid will boil when it is new. The wet boiling point indicates the temperature the fluid boils once it has absorbed water.

Brake Fluid is Lubricating

The antilock braking system (ABS) and calipers need to be lubricated to work right. Brake fluid has lubricating properties that keep the system working as it should.

Hygroscopicity in Brake Fluid

Hygroscopicity is a property that causes water attraction and retention. All but one variety of brake fluid (we will cover these below) is hygroscopic. This property is the main reason the brake fluid needs to be changed. Despite being an enclosed system, the fluid in the lines will still attract and retain moisture over time. Water in the brake lines reduces the boiling point of the fluid and can cause corrosion in the system, both of which can have negative impacts on your brakes. Brake fluid replacement is all about keeping water out.

Types of Brake Fluid

Brake fluid classifications begin with the acronym DOT, which is short for the United States Department of Transportation. These are the four categories of brake fluid you are likely to encounter:

• DOT 3
• DOT 4
• DOT 5
• DOT 5.1

You are probably wondering about the differences between them. Can you use different brake fluids interchangeably? In the next section, we will dive into the differences between each type of brake fluid.

What is DOT 3 Brake Fluid?

DOT 3 brake fluid is glycol based, and has a dry boiling point of around 400°F, and a wet boiling point of around 280°F. DOT 3 brake fluid absorbs less water than other types, meaning it needs to be changed less frequently.

What is DOT 4 Brake Fluid?

DOT 4 brake fluid is also glycol-based but has a higher boiling point than DOT 3. DOT 4 has a dry boiling point is 446°F, and a wet boiling point of 311°F. This makes it more reliable and consistent under heavy usage. The tradeoff with DOT 4 is it’s more hygroscopic, meaning it will take on more water and need to be changed more frequently.

What is DOT 5 Brake Fluid?

DOT 5 is the only non-glycol-based brake fluid variety and has a dry boiling point of 500°F and a wet boiling point of 356°F. Made of silicone, this type of brake fluid is typically used in classic cars, military vehicles, show cars, or any vehicle that sits a lot. Silicone is much more compressible than glycol, so the performance of the brakes is not as good. DOT 5 does a better job of protecting the system from the effects of water corrosion.

What is DOT 5.1 Brake Fluid?

DOT 5.1 has the highest boiling point of glycol-based fluids. It’s typically used in heavy-duty/racing applications. It has a dry boiling point of 500°F and a wet boiling point of 356°F, combining the high boiling point of DOT 5 with the superior performance of DOT 3 and 4. Keep in mind, DOT 5.1 is will retain water and requires frequent changes.

How Do I Know Which Fluid to Use?

If you bring your car to a shop for service, you probably have never thought about the kind of brake fluid you use. However, if you are in a situation where you need to top off your brake fluid, it’s important to know what fluid you need.

There are two ways to find out what brake fluid your car needs. First, you can look in the owner’s manual. The second way to tell is by popping the hood and looking at the cap on top of the brake fluid reservoir. On most vehicles, the brake fluid reservoir will be located in the engine bay directly in front of the driver’s seat and will have a yellow cap, or a cap with yellow lettering. The fluid rating will be printed on the cap.

Can I use Different Brake Fluid Interchangeably?

Here are some rules for brake fluid compatibility:

• DOT 4 can be used in a car that calls for DOT 3
• DOT 5.1 can be used in a car that calls for DOT 4 or DOT 3
• DOT 3 SHOULD NOT be used in a car that requires DOT 4 or DOT 5.1
• DOT 4 SHOULD NOT be used in a car that calls for DOT 5.1
• Systems that call for DOT 5 (silicone based) SHOULD NEVER use a glycol-based fluid, and they should never be mixed.

Brake Fluid Changes at Scott’s U-Save

The teams at Scott’s U-Save know how to service your braking system and keep your car stopping reliably. If it’s time for a brake fluid flush or other services, give us a call or schedule an appointment at our locations in New Lenox, Schererville, or Steger today!