Brake fluid is a vital component in ensuring the safe operation of your vehicle’s braking system. While there are different types of brake fluid available, DOT 5 brake fluid stands out as being quite different from the more common DOT 3 and DOT 4 varieties. In this article, we’ll explore what makes DOT 5 brake fluid unique and why it’s the right choice for some vehicles.
The Importance of Brake Fluid
Before diving into the details of DOT 5 brake fluid, it’s helpful to understand the role that brake fluid plays in your vehicle. Your brakes work by converting motion into heat energy through friction. When you press on the brake pedal, it activates a hydraulic system that pushes brake fluid through lines and into calipers that clamp down on your brake rotors. The friction between the brake pads and rotors converts motion to heat, slowing your vehicle down.
Brake fluid serves several key functions in this process:
- It transmits pressure from the brake pedal to the calipers
- It lubricates the internal components of the hydraulic braking system
- It absorbs the heat generated from braking and dissipates it
Given these important roles, it’s critical to use a high quality brake fluid that can withstand the demands of braking without breaking down. Next, let’s look at the key properties of brake fluid that affect performance.
Key Properties of Brake Fluid
All brake fluids are not created equal. The Department of Transportation (DOT) has specifications that brake fluids must meet, with higher numbers indicating more stringent standards. Here are some of the most important properties to consider when evaluating brake fluid:
Wet and Dry Boiling Points
Brake fluid absorbs moisture from the air over time, which lowers its boiling point. The wet boiling point refers to the boiling point of aged, moisture-contaminated brake fluid. The dry boiling point is the boiling point of new, uncontaminated brake fluid. Brake fluid that can withstand higher boiling points without vaporizing is superior, as vapor locks can lead to brake failure.
Viscosity
Brake fluid needs an optimal viscosity (thickness) to flow through brake lines and evenly transmit hydraulic pressure without compressibility. Very thick fluids may not transmit pressure well, while thin fluids are prone to leakage.
Compatibility with Rubber
Most brake systems contain rubber seals and gaskets that can be degraded by brake fluid over time. Fluids that are gentle on rubber components extend the life of the braking system.
Compressibility
Brake fluid should not be highly compressible, as compressible fluids result in a “mushy” brake pedal feel. Incompressibility allows for firm, responsive braking power.
Non-Corrosiveness
Brake fluid should not corrode or rust metal brake components like calipers, lines and valves. Non-corrosiveness protects the braking system from damage.
Now that we’ve covered the key brake fluid properties, let’s see how DOT 5 fluid stacks up in these areas.
Introducing DOT 5 Brake Fluid
DOT 5 brake fluid contains different additives than DOT 3 and DOT 4 that give it some unique advantages. Here are the characteristics of DOT 5 fluid:
Silicone Base
Unlike DOT 3 and DOT 4 which are glycol-ether based, DOT 5 is silicone based. The silicone provides a higher wet boiling point.
Higher Boiling Point
| Fluid Type | Dry Boiling Point | Wet Boiling Point |
|---|---|---|
| DOT 3 | 401°F (205°C) | 284°F (140°C) |
| DOT 4 | 446°F (230°C) | 311°F (155°C) |
| DOT 5 | 500°F (260°C) | 356°F (180°C) |
As you can see, DOT 5 has a significantly higher boiling point, both wet and dry. This allows it to withstand extreme heat without vaporizing.
Higher Viscosity
DOT 5 is around two times as viscous as DOT 4 fluid. The higher viscosity resists compression better under high temperatures and pressures.
Low Compressibility
The silicone base of DOT 5 makes it highly incompressible. This preserves braking power and pedal feel.
Non-Corrosive
DOT 5 does not corrode brake system components like other fluids can over time.
When to Use DOT 5
Now that we’ve compared the properties, when is it appropriate to use DOT 5 over other brake fluid types?
DOT 5 is recommended for a few specific applications:
- Race Cars – Race cars require high temperature performance. DOT 5’s exceptional boiling point and compressibility make it ideal for race conditions.
- High Performance Sports Cars – Sports cars with large brakes and sticky performance tires generate lots of heat when braking. DOT 5 is well suited for high performance street driving.
- Show Cars – For show cars that sit idle for long periods, DOT 5 is less likely to absorb moisture than glycol fluids, preserving the braking system.
- Older Vehicles – DOT 5 is compatible with rubber components in older brake systems that may have issues with glycol-based fluids.
For normal passenger vehicles, DOT 3 or DOT 4 fluid is usually recommended, as they have better cold weather performance due to lower viscosity. But for specialized applications, DOT 5 is an excellent solution.
Downsides of DOT 5
DOT 5 brake fluid isn’t necessarily better across the board. There are a few downsides to be aware of:
- Higher viscosity can mean longer brake pedal response times in cold weather.
- Not compatible with some glycol-based fluids – systems should be flushed before switching.
- More expensive than DOT 3 or DOT 4.
DOT 5 should not be mixed with glycol-based fluids, so a full flush is required to convert systems over to silicone fluid. The higher cost and unique properties of DOT 5 also mean it’s not ideal for routine use in most vehicles.
Maintenance Tips
All brake fluids absorb moisture over time, lowering boiling points. Here are some tips to maintain your brake fluid quality:
- Flush your brake system and replace fluid every 2-3 years.
- Use only new fluid from a sealed container, don’t top off with old fluid.
- Inspect fluid color – it should be light and transparent, not dark and murky.
- Replace rubber seals and gaskets whenever changing fluid.
Sticking to these basics will keep your braking system in top shape.
Conclusion
DOT 5 brake fluid stands apart from more common DOT 3 and DOT 4 fluids thanks to its silicone base. The high boiling point, viscosity and compressibility of DOT 5 make it ideal for race cars, high performance sports cars, show vehicles and older braking systems. For normal driving, glycol-based fluids are usually sufficient, but for specialized applications, DOT 5 delivers superior high heat braking performance. By understanding what makes DOT 5 different, you can choose the right fluid for your vehicle’s needs.