QUESTION: I am about to change the rotors/pads on my Mini Cooper and have a question about old brake fluid. I have a liter can of ATE SuperBlue that I opened four or five years ago, but used only a few milliliters. It has been tightly capped and unopened during this period. Should I use it or should I properly discard it? I bought a new can — for $20! — and planned to use this first and the old can next, if needed. What is the lifespan of tightly capped brake fluid? What do you recommend?

—Charlie Cump

ANSWER: Charlie, you’re wise to ponder this. Renewing brake pads and rotors may actually require you to remove some fluid rather than add it; pushing the caliper pistons back to make room for thicker pads pushes fluid back to the reservoir, possibly overfilling it. I’ll assume you also plan to flush and refill the brake system’s fluid, which is good to do on a routine basis.

Brake fluid is hygroscopic, meaning it readily attracts moisture. This sounds crazy, but moisture can enter a brake system through its flexible rubber hoses and rubber seals at a rate of about 1 percent per year. This can become a problem in an automotive brake system for two reasons.

First, moisture has a much lower boiling point than brake fluid. Brakes convert movement's kinetic energy into significant heat, and some of it passes to the fluid. If brake fluid is "wet," meaning typical DOT 3-grade brake fluid with 3.7 percent suspended moisture, it will begin to boil if its temperature rises above 284 degrees F. This is a huge problem: The vapor bubbles caused by boiling are compressible, which can cause a dangerous loss of hydraulic force and braking action. New or "dry" DOT 3 brake fluid has a much higher 401-degree boiling point, and DOT 4 brake fluid has a boiling point of 446 degrees. That makes a very good case for routine brake fluid replacement.

Secondly, moisture, the reaction of various metals used in the brake system parts, and depleted corrosion inhibitors will lead to corrosion issues within a vehicle’s precisely fitting and expensive ABS brake system components over time, as well as other brake system parts.

Your pricey but very cool ATE SuperBlue fluid is a high-performance racing brake fluid that exceeds DOT 4 specifications with dry and wet boiling points of 536 and 388 degrees respectively, a significant improvement over generic fluids. Now the sticky part — shelf life. Among manufacturers, 2-3 years seems to be the typical recommended shelf life for a new, still-sealed brake fluid container, and logically an opened and resealed container would be less. For this reason I have to say your fluid should not be used.

Test strips are available to check brake fluid within a vehicle’s master cylinder fluid reservoir, which should be done annually, or perhaps to check a can of brake fluid you are considering using. Better test strips containing FASCAR technology check not only for moisture level but also the effectiveness of remaining corrosion inhibiters. These have been shown to be significantly depleted in as little as 30 months of use. Color is not a reliable indicator of good or bad brake fluid.

Periodic brake fluid replacement is, oddly, not mentioned by some vehicle manufacturers in their maintenance recommendations while others and most service industry folks (including myself) recommend doing so every two to three years. A vehicle driven mostly around town might never incur brake temperatures high enough to be a problem, but those used for towing or that see demanding braking conditions could come close to the danger zone with "wet" fluid.

My take on a 2002 study by the Automotive Maintenance and Repair Association on brake fluid renewal suggests that corrosion due to depleted fluid additives is just as important as temperature-induced braking system failures. Flushing and refilling used to be a straightforward job within a home mechanic’s abilities, but modern ABS brake systems can complicate the process. It’s best to leave the job to a pro.

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