I have a small, 12-ounce bottle of DOT 4 brake fluid that’s been on the shelf for at least a couple of years. I was going to use it but two things got my attention. First, the plastic bottle had collapsed–almost like it was sucked in. That made me wonder if the fluid inside had somehow changed. I know that brake fluid is supposed to be kept in a sealed container to prevent moisture contamination. This bottle was brand new with the sealed anti-tampering cover still in place under the cap. Secondly, when I opened it, the fluid wasn’t clear–it was an amber color. I immediately assumed it was bad because it was nearly the same color as the fluid that I was replacing in my Mustang after I changed front brake pads. Is there a shelf life for brake fluid?
B.C.
Jeff Smith: This is a great question. Let’s first answer your questions and then talk a little more about brake fluid because there’s a ton of useful information. It seems you already know a little bit about how brake fluid absorbs moisture directly out of the air. This makes the fluid hygroscopic. Anytime the brake fluid is exposed to the atmosphere, it will immediately begin to absorb water out of the air. Obviously, if you live in a humid part of the country like Houston or Miami, this is a big issue. We called a couple of the brake fluid companies and the answer was two to five years is a safe number for an unopened container. Wilwood’s tech line claims that if the container is sealed, they feel it has an unlimited shelf life, but I would consider five years to be the limit. Once the container has been opened, the shelf life is going to be much shorter. The lower the level of fluid in the container, the more air that is present and therefore more moisture that will absorbed into the fluid. That’s why there is a rubber bladder between the metal lid on the master cylinder and the fluid.
Why did the container collapse as you describe?
I have seen this before and the company I spoke to about this admitted that they had been using a thin wall plastic container that deformed after the fluid was sealed but it does not mean the fluid has failed. Finally, the amber color should not be of concern because many brake fluid manufacturers now color their DOT 4 fluid with an amber color. This is normal and is not an indication of contamination or that it is somehow tainted.
Moisture contamination is a critical issue because it directly affects the brake fluid’s boiling point. For example, the standard passenger car DOT 3 fluid dry boiling point is 401 degrees F. The Federal Motor Vehicle Safety Standards (FMVSS) define a wet brake fluid as containing more than 1.5 percent water. At this point, a DOT 3 fluid’s boiling point has dropped from 401 to 284 degrees F. It’s also likely that many older street driven cars might have water contamination well above 10 percent. Consider that street driven brake rotors can see temperatures in excess of 500 degrees F and racing brakes (those red hot rotors on road race and NASCAR short track cars like at Martinsville) can see rotor temperatures of 700 to 1,100 degrees F. Even at those lower 500 degree temperatures, any water in the brake system will boil. When the brake fluid boils, it creates bubbles in the hydraulic system that is designed to work only with a nearly incompressible fluid. With air in the system, it compresses and the brake pedal becomes spongy or just flat goes to the floor–not what you want under any circumstances.
Because racing brakes operate at such elevated temperatures, there are racing fluids such as Motul, Castrol SRF, Wilwood EXP 600-Plus, and others. These DOT 4 fluids offer a minimum dry temperature of 446 degrees F. These fluids operate at much higher temperatures without boiling but this also comes at a price. Yes, the fluid costs more, but it also is more likely to absorb moisture out of the air more readily than the DOT 3 fluid. So that means the fluid must be changed more often.
So how do you know if your fluid has been contaminated?
For a street driven car operated in a humid environment, exchanging the brake fluid every three years would be a good maintenance program that would certainly promote excellent hydraulic circuit performance. Unfortunately, few enthusiasts will even think about doing this. For a performance car with big, multi-piston brakes that is used aggressively on track days and autocross courses, a brake fluid flush at the beginning of the season is an outstanding idea combined with a fresh bleed midway through the season. You might find that bleeding before every event is a good idea, especially if it promotes a nice, firm pedal. When you consider how much money some of these big brake systems cost, a little money spent on brake fluid is just good insurance that the brake pedal will always be firm and high. (There’s a line from a Bob Seeger song that mentions firm and high, but I don’t think he was talking about brake pedals.)
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