I have a couple of containers of open Super Blue and ATE Type 200 brake fluid. I've been told that once opening the container, one should use the fluid within a couple of days. It was my understanding that as long as moisture doesn't get in, it should be fine, but I've had multiple people tell me that I shouldn't use what I have already open, as it's been sitting for a while, perhaps a month or two. I have my container of Super Blue with the cap on tight, in a zip-lock bag with it zipped fully. Is this sufficient to prevent moisture buildup? Or, should I consider this a wasted container? I asked someone at Fairfield County Motorsport (a CVC approved tech inspector) and one of the guys told me that as long as I keep the cap tight, I can use it up to a year later. Is there any validity to this? What's the final word on whether or not I can use this fluid?
Ask ten people and you'll get twenty different opinions... The deal with all brake fluid other than DOT 5.0 which is silicone based and only recommend for use in museum type/stored vehicles, is that brake fluid is hygroscopic, which means it absorbs moisture from the air. The absorption of water can lead to corrosion of the brake components over time and to a spongy brake pedal or complete loss of brakes if your brake fluid gets really hot like at track driving HPDE type events and boils. For all practical purposes when brake fluid is in a sealed container it can't absorb moisture other than what is in the container when it's capped. Yes plastic containers can allow a minute quantity of air to enter over a long period of time like years, but not so much water vapor. Since the brake system in all cars is open to atmosphere - it can absorb moisture out of the air easily, unlike a capped container of brake fluid. That is the reason why all brake systems should be flushed every two years per the OM and why brake systems should be bled or flushed prior to every HPDE track event where the fluid will see extreme heat. So the long winded answer to your question is IMO and experience that properly capped brake fluid can be stored and then used without issue for a year. It probably could be stored for several years and still be fine as far as how much moisture it has absorbed, but it's better to error on the safe side. Days or weeks is just silly unless the cap has been left off the container. Then I would discard it. In addition ATE 200/Blue is very reasonably priced so tossing any leftover every year is not a financial issue IMO, but it should be just fine for a year if properly capped. I doubt zip lock bags are 100% air/moisture proof but it makes it harder for that nasty water moisture to get in to the container.
It is fine for a long time with the cap closed. Moisture can only get into a container with no cap and there is only so much moisture in the air above the fluid when you cap it.
I don't store opened brake fluid at all. When I flush my brake fluid it takes almost a liter. Even swapping new brake pads to worn track pads, I have never had to add brake fluid. I have never had the need to top of brake fluid when swapping brakes. Maybe if I was using brake fluid costing $75.00/liter, I'd think twice about discarding it. But at ~$10.00/liter, I'd rather discard it and leave no question in my mind.
That's all fine and well but some folks have fluid left over from a brake bleed if they do occasional HPDEs and there is nothing wrong with properly storing the remaining fluid. While ATE is quite reasonably priced the Motul RBF600 and similar are more expensive. Some brake fluids are available as .5L containers and others like the ATE are more commonly in 1 liter containers. There is no question with capped brake fluid.
