Brake fluid is one of the many liquids that modern cars need in order to run (or stop). It's a type of hydraulic fluid with a high boiling point, but it also absorbs moisture like a sponge. DOT 3, DOT 4, and DOT 5.1 are hygroscopic fluids that absorb moisture in the braking system and provide lubricity to combat rust and corrosion. In comparison, DOT 5 is hydrophobic, the exact opposite of DOT 3 or DOT 4 fluid. DOT 5 fluid uses a silicone-based formula and does not absorb water or moisture. However, most cars on the road today use DOT 3 or DOT 4 brake fluid, so we'll focus on those regarding fluid lifespan.
The moisture-absorbing, hygroscopic properties of DOT3 and DOT 4 brake fluids mean that the fluids' shelf lives will depend on storage methods and air humidity. Brake fluid will start absorbing moisture immediately after you open a fresh bottle and break the seal. If brake fluid absorbs too much moisture, its boiling point will drop. For context, Valvoline DOT 3 brake fluid has a dry boiling point of about 495 degrees Fahrenheit and a wet boiling point of 315 degrees Fahrenheit. The second number refers to the boiling point after the fluid has circulated through the system and absorbed moisture.
As you can see, brake fluid contaminated by moisture has a much lower boiling point than dry, fresh fluid, and that's bad for your car's brake system. It's also why you should use open bottles of brake fluid within three to 12 months. Even if the fluid still looks and smells fresh, we don't recommend using it if the bottle has been sitting around for over a year.
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The fluid circulating inside your car's brake system won't last forever, and periodic flushing services are necessary to deliver a reliable braking performance. The brake fluid degrades as it absorbs moisture, so you should change it every two years or 30,000 miles, though your actual mileage can vary. Check the owner's manual to learn the required service intervals for your car's brakes.
It's a good idea to inspect or replace the brake fluid whenever you change the brake pads. Moreover, bleeding air out of the brake system at least once a year will help improve braking performance in the long run. When adding brake fluid to the reservoir, you're better off cracking open a new bottle. Pouring contaminated brake fluid into the system will only give your car a spongy brake pedal and weaker stopping power.
When in doubt, insist on a new bottle of brake fluid instead of hedging your bets on fluid that's been hanging around for an unknown period of time. Moreover, take care when handling brake fluid. It is highly corrosive and can quickly ruin your car's paint. Luckily, if you spill brake fluid on your car's paint, you can save yourself an expensive paint job by acting fast and cleaning up the mess before permanent damage occurs.
