How Often Should You Flush Brake Fluid?

How Often Should You Flush Your Brake Fluid?
From the left: New vs Old DOT 3, New vs Old DOT 5

It must be week here at Ed’s BnB! First, we tackled the question “Do I Need a Brake Fluid Flush?”, And explained why it’s a good idea. Now, we’re going into Should You Flush Your Brake Fluid?

As we covered in the article linked above, most cars on the road today use DOT 3, 4, or 5.1 brake fluid. These brake fluids are hygroscopic, which means they have a tendency to actively absorb moisture content from the humidity in the air.

Your brake system isn’t completely sealed. There’s a tiny hole in the cap on the top of your master cylinder. This tiny hole is necessary to allow the to work. However, it also means that every time you press the brake pedal, a small amount of air circulates into the master cylinder reservoir.

Over time, your brake fluid will absorb the humidity from that air. As the moisture content of your brake fluid rises, it’s boiling point falls. Since your subject the fluid to some pretty extreme temperatures, you want to be sure that it’s not going to boil.

If brake fluid (or trapped moisture) is allowed to boil in the caliper or lines, your could become weak or fail completely. This is because the resulting vapor is compressible, unlike the fluids themselves.

Should You Flush Your Brake Fluid?

You should get a every 2 years, or 30,000 miles. Whichever happens first. By that time, your brake fluid will likely be approaching around 4% content. That amount of moisture is enough to begin to corrode your braking system components. If it isn’t flushed out, anyway.

If you happen to have a vehicle that uses DOT 5 (silicone), you’re in luck. You can go longer between flushes since it doesn’t absorb as readily.

If you’re not comfortable tackling a yourself, give your local brake shop a call!

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