If you’re like most car owners, you’ve probably asked yourself at some point, “Is brake fluid universal?” Maybe you were topping up your fluid, browsing the aisles at an auto parts store, or shopping online and noticed labels like DOT 3, ドット 4, and DOT 5.1. It’s easy to assume brake fluid is one-size-fits-all—but the truth is, it’s not.
この記事では, I’ll share everything I’ve learned about brake fluid compatibility, what those mysterious “DOT” numbers mean, and how to make sure you’re using the right fluid for your vehicle. This article is written from a real car owner’s perspective—just like yours—and is designed to clear up all your confusion in one place.
What Makes Brake Fluid Different?
All brake fluids are not created equal. Brake fluid types are classified mainly by their chemical composition and boiling point, and that has a big impact on performance and compatibility.
Here’s a quick comparison:
| Type | Base | Boiling Point (Dry/Wet) | Compatible With | Common Use Cases |
|---|---|---|---|---|
| ドット 3 | Glycol-based | 205°C / 140°C | ドット 3, ドット 4 | Most passenger vehicles |
| ドット 4 | Glycol-based | 230°C / 155°C | ドット 3, ドット 4, ドット 5.1 | Performance and modern vehicles |
| ドット 5 | Silicone-based | 260°C / 180°C | Not compatible with others | Classic cars, military vehicles |
| ドット 5.1 | Glycol-based | 270°C / 190°C | ドット 3, ドット 4, ドット 5.1 | ABS systems, high-performance cars |
Why Brake Fluid Isn’t Universal
それで, why can’t we just use the same brake fluid in every car?
1. Different Chemical Bases
Some brake fluids (like DOT 5) are silicone-based, while others (ドット 3, 4, そして 5.1) are glycol-based. Mixing silicone and glycol fluids can cause system failure, corrosion, and rubber seal damage.
2. Boiling Point Requirements
High-performance or newer vehicles often need a higher boiling point to handle heat generated by ABS and aggressive braking. Using a lower-grade fluid like DOT 3 in a car that requires DOT 4 or DOT 5.1 could lead to brake fade or failure under pressure.
3. Water Absorption (Hygroscopicity)
Glycol-based fluids are hygroscopic—they absorb moisture over time, which lowers boiling point and affects performance. ドット 5 (silicone-based) doesn’t absorb water but may trap air bubbles, making it risky for modern braking systems.
Can I Mix Different Brake Fluids?
Here’s a quick rule of thumb:
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ドット 3, ドット 4, and DOT 5.1 are technically mixable because they’re glycol-based—but it’s not recommended unless you’re in an emergency. Always stick with what your vehicle manual specifies.
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ドット 5 is not compatible with any of the others. Never mix it.
Even if mixing is possible, it can reduce the overall performance of your braking system. If you’re unsure what’s already in your car, it’s best to flush the system and start fresh with the correct fluid.
How Do I Know What Brake Fluid My Car Uses?
The best source is your owner’s manual. If you don’t have it handy, check under the hood. Most brake fluid reservoir caps will specify the required DOT number.
Also, consider:
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Driving conditions: Aggressive driving or hilly areas may benefit from higher boiling point fluids.
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Climate: Wet or humid environments increase the rate of water absorption, so you may want a more stable option like DOT 4 or DOT 5.1.
避けるべき一般的な間違い
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Assuming all brake fluids are the same. This is dangerous and can damage your braking system.
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Mixing incompatible fluids. Especially DOT 5 with others—just don’t do it.
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Overfilling the reservoir. Brake fluid expands as it heats up. Overfilling can cause leaks.
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Using old or opened fluid. Brake fluid absorbs moisture from the air. Always seal the bottle tightly and use it quickly.
When Should I Change My Brake Fluid?
Even if your brakes feel fine, old fluid can corrode internal parts and reduce stopping power. A general recommendation is:
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Every 2 年 または
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Every 20,000–45,000 miles
(Refer to your manual for specifics)
よくある質問
Q: Can I use DOT 4 instead of DOT 3?
あ: はい, ドット 4 is compatible with DOT 3 and has better performance. しかし, don’t use DOT 3 in a system that requires DOT 4.
Q: Is DOT 5 better than DOT 4 or DOT 5.1?
あ: 必ずしもそうではありません. ドット 5 has a higher boiling point but is silicone-based and not compatible with modern ABS systems.
Q: What happens if I use the wrong brake fluid?
あ: You risk damaging seals, reducing brake performance, and even total brake failure. Always use the correct type.
Q: Is DOT 5.1 just an updated version of DOT 5?
あ: いいえ. ドット 5.1 is glycol-based and compatible with DOT 3 そして 4. ドット 5 is silicone-based and not compatible with the others.
Q: Can I tell what brake fluid is in my car just by looking at it?
あ: いいえ. Most fluids are clear to amber. Always go by what’s specified in the manual or label.
最終的な考え
Brake fluid is not universal—and assuming it is can lead to serious problems. Always check your car’s specific requirements, and if you’re ever unsure, play it safe by flushing the system and refilling with the recommended fluid.
A few minutes of checking today can save you from brake failure tomorrow. Stay safe—and drive smart.
