Life Expectancy of Brake Fluid

[Norman]

Not really sure if this is the correct place to post this, but I need to start somewhere.

My question is:
Once you open a container of brake fluid, and break the seal on top, How long does the fluid last in the container before it needs to be discarded?

The reason for this question is that every time I need to top off or replace the brake fluid on some vehicle, I always purchase a new bottle. I always date the bottles when I open them, but have no idea how long I can keep them and use them safely. It is now hazardous recycling time in my area, and I would like to get rid of what is no longer any good and keep what is still usable. I usually won’t use an used but closed container of fluid after about a month after the date it was opened. Am I being overly cautious and wasteful in my thinking or not cautious enough?

I gratefully appreciate any and all opinions.
Thank You
Norman

Topic

[Rob Jorgensen]

How often do you replace your brake fluid? I would let that be your guide. Brake fluid goes bad by absorbing moisture (hygroscopic) and it does that in the bottle as well as the brake system.

[Norman]

Thanks for your reply grouchy-hermit. I personally change out my brake fluid around every two years, however I can not go by that. I do general maintenance on a couple of limo fleets as well as on family, friends and neighbors vehicles, totaling to around 100 vehicles in all. I also take care of most brake system problems. I am not a trained mechanic, but a diy’er that happened to pick up some side work that turned into something more.

I personally would not use brake fluid that is two years old once the seal is broken, I wouldn’t even use it after a year. So my question still stands, How long is a container of fluid good for once the seal has been broken. At this time, I won’t use a bottle that has been opened longer than a month, only because I don’t know what the shelf life is after the seal has been broken. If the fluid is still safe to use six months later, then I won’t have to by so much and waste it.

[Rob Jorgensen]

If you are comfortable with brake fluid being in your vehicle for 2 years, then that should be your guide on opened containers. The fluid in the opened container should deteriorate at about the same rate as that in the vehicles, if not slower.

According to Valvoline FAQs:

“Any chemical that has been opened, including brake fluid, power steering fluid, etc., has a recommended shelf life of no longer than two to three years, depending on storage.”

[Hokiedad4]

It would be an interesting study to see how much water is absorbed by brake fluid in its bottle, both before and after the seal is removed. Many fluids come in HDPE bottles, which is considered a semi-permeable container. So some moisture can pass through the plastic. However, it’s very slow. Brake fluid fortunately is pretty cheap, so there’s no reason to use old bottles that have been sitting around.

[subhobroto]

Off topic, but do manufacturers mention the amount of brake fluid in the reservoir and master cylinder?

My owner or FSM does not mention it, and neither does those of the cars I have helped out on, but I am not a professional mechanic, so my exposure is very little.

Is it atypical for manufacturers mention the amount of brake fluid in the reservoir and master cylinder?

[David]

system capacity is usually in the service manual, and sometimes the owners manual. It’s actually irrelevant – just fill it until you are at the full line when you replace the pads (note – the fluid level will drop as the pads wear and the cylinders extend in the calipers – if you top it off, then change the brake pads and push the cylinders back, you’ll overflow the master cylinder and make a mess)

In terms of brake fluid life, water uptake is the issue – most fluids (with some exceptions, like ATE superblue) will change color as they take on water. How long they last really depends on where you are located – when I lived in VA, the fluid would get really dark inside a year. Down here in the desert of NM, it takes a long, long time to take on water – I’ll change the fluid when I change pads (usually every 2-3 years on the wife’s van) and it’s just barely darker than new in the reservoir. Note that the fluid you really care about is what’s in the calipers – as the fluid takes on water, the boiling point falls (not really an issue for most street cars) but more importantly, it gets dramatically more corrosive, and can attack the cylinders and the bores in the caliper, making it so that the caliper can’t seal and will leak (requiring either a rebuild or replacement)

[Author]

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