Why can't you make use of an opened container of brake fluid even if it has been closed tightly when the brake fluid reservoir cap on your automobile doesn't appear significantly more airtight?
Is there something about the brake fluid reservoir that makes it more moisture resistant than the plastic cap on the bottle of brake fluid? Update: I realize brake fluid is hygroscopic and how that absorbed water causes braking problems. My concern is why does the brake fluid in the opened brake fluid plastic bottle with the lid closed tight absorb more water over moment compared to the brake fluid in the brake reservoir?
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If it was recently opened, you can. But let's say you replace brake fluid every 2 or 3 years. You wouldn't want to use the leftover fluid from the last change. It could be just as bad as the fluid in the system.
“Theoretically” as bad, the fluid in the car is likely to be worse
I don't see how. The brake system has to be vented so it can see humid air. A sealed bottle doesn't. Since folks seem to disagree, I googled a brake fluid reservoir is vented. The reservoir cap is designed with a vented system to allow air to enter or exit as the brake ...
>Why can't you use an opened container of brake fluid even if it has been closed tightly Says who? I'm not throwing away a 7/8 full quart bottle of brake fluid after topping off a reservoir and having the cap off for 15 seconds.
Why are you topping off fluid in a closed system? If you just bleed them, sure. But I hate it when people just top it off when in reality it is because their pad life is almost gone. Now, when the calipers are reset, the res is going to overflow. Lol.
Yup my bottle is 5 litres. It last years.
Get a moisture meter. It may or may not be ok. I have used opened containers of brake fluid. They were closed tightly and i checked the moisture. Brake reservoirs cannot be sealed because they have to let air in as the pistons extend due to brake wear.
Most reservoirs use a bellows type gasket to seal and allow volume change without directly venting
Wow. Some people are over fussy. We sure use up the left over brake fluid in the next couple of years , with no ill effects
I know man, lots of google mechanics and not real life mechanics in this sub.
That doesn't mean it's inherently safe. You can drive without a seatbelt for years and be fine, but the one time it's not... Well, good luck. Brake fluid absorbs water out of the air. Water has a significantly lower boiling point than brake fluid. If you're in a situatio...
It doesnt. An open container will last about 2 years. Approximately The same amount of time the fluid in your reservoir will last. We recommend brake fluid changed every 2 years. This is what all chrysler vehicles are recommended at by the manufacturer. This is mostly to...
A liter of air at 50% humidity has 0.00865 mL of water. If you have a 1L bottle of fluid and you use half of it, and when you seal it back up you're sealing half a liter of air in there so ~0.00432 mL in 500mL of fluid. That's 8% of a drop of water in half a liter of water.
Maybe for a car you want to flush with brand new fluid but I drive older vehicles and repair things as they go. Topping off with an opened bottle thats been screwed on tightly on the garage shelf doesn't bother me at all. I just changed a caliper that went bad and then topped ...
I little water in the system on a standard car isn't a big deal till it starts rusting parts. A little bit of water in a race car can boil and cause you to lose brakes more quickly, or at least that my understanding of it. Kind of like how drum brakes are fine on a classic c...
Who said that a sealed container is less airtight than the reservoir? It's not. But once the brake fluid in an opened bottle contacts atmospheric air, it absorbs moisture. Is that worse than what's in the car? Heck no, a lot of older reservoirs even have a hidden breather. The...
How much water though? A liter of air at 50% humidity has 0.00865 mL of water. If you have a 1L bottle of fluid and you use half of it, and when you seal it back up you're sealing half a liter of air in there so ~0.00432 mL in 500mL of fluid. That's 8% of a drop of water in ha...
Its absolutely fine. Stores want to sell more fluid so they find patsys to sell the extra shit to. No one wants to be the only patsy so they just repeat the same bullshit story that they fell for.
I found a solution to any argument. I have a slow leak, so I fill my reservoir every two weeks or so. My bottle never gets old enough to be of concern.
Brake master cylinder reservoirs typically have a rubber seal inside the cap, it is made in such a way as to fill the space in the reservoir as the brakes wear to keep air away from the fluid. The reason not to use old opened brake fluid is that brake fluid is hydroscopic an...
Then that same amount of air and moisture is introduced into the fluid when you pour it into the master cyl reservoir. Unless you're using a vacuum system and somehow never expose the fluid to air.
I've never had an issue with using opened containers of brake fluid hell I used to store a bottle of it under the hood
You’re right about the brake fluid bottle being far more airtight than the brake reservoir on the car. Some even have a pinhole to let air in; they need a way to handle the pressure changes in the reservoir. It’s not remotely airtight. It’s more of a best practices thing. Th...
Do an amazon search for "Phoenix Systems 8006-B Double-Ended Brake Fluid + Coolant Test Strips" Will tell you condition; https://preview.redd.it/20c5hif8soog1.jpeg?width=799&format=pjpg&auto=webp&s=22486cb8fa4dbf198e06ac9e2438df6469e50aba
It’s the double ended thing your significant other won’t look at you funny for searching
You *absolutely can* use brake fluid from an open container-*for a year or so(YMMV). it's still fine until it noticeably darkens, or gets cloudy-that's when it's contaminated-just like the fluid in automobiles. That's why you should change out the brake fluid every few years.
This is also why I try to always use the amber brake fluid. If it goes bad it will usually change colour.
Call me OCD, but I do my own brake fluid flushes on my vehicles every 2 years or so. If you've ever bought a used vehicle 5+ old that's been driven and it's never been done, and you flush the fluid through with new, you'll get a noticeably firmer and consistent pedal when brak...
The reservoir actually isn't more airtight, you're right about that. The difference is volume ratio. In the reservoir there's usually very little air space above the fluid because it's nearly full, and that small air pocket doesn't hold much moisture even if it exchanges with ...
Because you let air in the bottle when you open it and when you put the lid back on it you lock the air with moisture in the bottle. So therefore since you know it’s hygroscopic then you know it absorbs the moisture from that air. Plus the lids don’t seal that well or they wou...
The seal is in case the lid comes loose in shipping. Go squeeze an open bottle of brake fluid with the cap screwed on tightly; if it will not leak air when you're squeezing it, it won't suck in under atmospheric pressure. A liter of air at 50% humidity has 0.00865 mL of water...
I wasn’t aware of this idea, I know brake fluid absorbs water, but I’ve used left over dot4 that might have been a few years old in clutch and brake lines on bikes for 40+ years without any issue.
Dot 4 hasn't been around that long
You should replace the fluid in your car every 2 years - brake fluid is relatively cheap while ABS pumps, master cylinders and calipers are not. Likewise fluid in a bottle is ok for about the same amount of time. Adding old fluid to old fluid is a great way to make sure there...
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I do use it. np ime but then the opened ones isnt open for years.
I don't think a couple years matters. The bottle I have that I haven't touched in ten years... should probably throw that out.
Better to not take the chance. Brake fluid is cheap in the grand scheme of things, just buy new.
This. Break fluid is not very expensive. Buy the small bottle if you do not need to flush lots of brakes. There isn’t a whole lot of fluid in the whole system!
In repsonse to your edit... It doesn't absorb *more* water compared to your reservoir. It's the fact that it absorbed water period. Then you pour it in your reservoir and it's already starting off at a lower boiling point than a fresh bottle and then it's just going to colle...
I think the rule is a suggestion for people who are not good at understanding what they are doing. Moisture will not get past a tightly fitting lid. Moisture only gets into fluid in the reservoir via its atmospheric venting. No moisture gets in at the calipers..The concern fo...
Short answer: It doesn't if you close it properly and if the bottle is designed well. This is just a myth people are taught that I am yet to see any evidence of. I've have 2 big 4L bottles (alternating colours) that lasts me around a year or two and never had an issue. If yo...
I personally think it's fine if it was tightly capped. For me. But not for you because if you didn't cap it tightly and you live if a humid area and then you use it and happen to die in a horrible fiery car crash caused by brake fade, none of us nor our lawyers want your next...
My two cents: if the opened container is metal it will keep the brake fluid fresh longer than a plastic container. With any container though make sure it is tightly resealed once opened.
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Brake fluid just has a “life cycle” that starts the moment it’s exposed to air. Some reservoirs are more air tight than the cap on a bottle and some are vented. It’s just apply the logic of its life cycle. If your replacing 2 year old fluid in a car with an 2 year old o...
Brake Reservoir is sealed, it's just not a hermetic seal. Nor is the bottle cap once you've removed the glued on seal, it is sealed and not hermetic. Brake Reservoir requires ventilation for multiple reasons, all relating to thermal expansion as it pertains to chemical reacti...
i needed this today. i was about go get brake new brake fluid as opposed to using the one i bought new a month ago
I wouldn’t use an open container of brake fluid that was years old, but I would and have used it when it was open for say 6 months with no ill effects. My 2006 Accord had the original brake fluid as far as I know when I sold it in 2023 with 410,000 miles. The brakes worked gre...
Because brake fluid is hygroscopic. Meaning it will absorb water. You dont want water in your brake system.
Ive always wondered this too. Seems like one of those rules mechanics repeat but nobody really follows. If I closed it tight and its only been a few months im using it. Cant be worse than the 5 year old fluid already in the car probably.
Brake fluid is hydroscopic meaning it absorbs any water in the air. If the bottle isn't very old like a month or two maybe it should be fine but even over time the fluid can break down that's why they recommend flushing your brake fluid at certain intervals. Same as engine oil...
I use them.
Opened brake fluid has a shelf life of 2 years max. The brake fluid in your car should be changed every two years according to manufacturers. Does this get ignored by the majority owners and they don’t suffer issues, also true. I would use my old bottle to top my pickup, but t...
New cars could have more issues with water in the brake system effecting the antilock system. But changing brake fluid every two years is just a ridiculous rouse by car dealership to make money. You can easily test the fluid for water . The dealer doesn’t check it for water th...
It's because brake fluid is "hygroscopic" meaning it absorbs water naturally from the air. It's recommended to change every 2 years but most cars never see a brake fluid change unless there's been a repair to the braking system where there's been a loss of fluid. It's probab...
I tested my five year-old bottle of Bosch 5.1 and it indicates zero moisture. I’ll be using it soon
Because a tight shoddy plastic lid isn’t air tight. Sit that bottle upside down with the seal removed and watch fluid find its way out within days.
How old? Eventually it'll take on water and be less effective, because the seal is imperfect. Depends a lot on humidity. After ten years, the brake fluid in my car is still fine. But in other situations, it might not be. Same with brake fluid on the shelf. But it is chea...
If it’s less than 3 years old, your fine. Even on some more “modern” cars, I’ve seen a (very few) vented brake fluid cap. It was a 02 Hyundai. And it appears the fluid had never been changed. One wheel cylinder is bad. And it was black. I buy by the gallon, have 4 of my own ca...
As long as the new fluid is still clear I'll use it. If it's apple juice color it goes in the waste drum.
So they have convinced people that brake fluid goes bad lol this is wild.
I have boiled my brake fluid before. It's not a myth.
I literally opened a master cylinder today that looked like a dog took a shit in it . Brown from sitting for 5 years in Maryland.
It does. Source: the laws of physics and chemistry. It’s formulated to be an incomprehensible fluid with specific viscosities at various temperatures. That chemical make up results in a hydroscopic fluid. When it unavoidably absorbs water from the environment the chemical ...
Brake fluid absorbs moisture from the air. Using an open bottle might cause moisture to get into your braking system
I’m a mechanic of 29 years. I’ve never changed the brake fluid in any of my cars and I’ve never had a problem with the brakes.
It’s hydroscopic. It slowly absorbs moisture from the air. That moisture will rust your system over time.
It doesn’t rust the system , it solidifies the fluid over time . 
Unless you store it in a container under a vacuum which removes all the moisturized air in it like a wine pump. It's going to get contaminated with moisture. The less fluid in the container the more contamination.
Because you are essentially putting in old fluid. It’s already partially used up before it ever gets inside your brake reservoir.
Brake fluid is what's called "hygroscopic" which means it absorbs moisture from the atmosphere around it. Because hydraulics function on the principle that fluids cannot be compressed, old brake fluid can fail to perform as expected due to absorbing gases within the atmospheri...
A bottle of brake fluid is $5-6. Sure, I’d use a bottle that had been recently open. No, I’m not going to use one that was opened 2-3 years ago. I’d just buy a new bottle or two. I’m not homeless. I can afford it.
I have the same brake fluid in my 79 Mercury that was put in at the factory. Still works just fine
Because bake fluid is hygroscopic. It absorbs moisture through the air bringing the brake fluids boiling point down. That in turn causes brake fade etc…
I put 15 years old brake fluid in my Lexus RX350. They look fine to me.
Looks fine and is safe are two different things. The boiling point for brake fluid drops significantly when it absorbs a lot of water, you're running the risk of boiling the fluid in severe driving conditions which can cause brake failure
It's hygrscopic. What more do you need? Ir use it and hopefully nothing will rust and/ir the fluid won't boil that one time you really needed to stop very quickly.
You really didn't address what he asked. Nice
Can go 20 or more years without changing brake fluid, or brakes for that matter, lol
You can, sure. An old vehicle with no ABS? Yep good chance everything will still work under non stressful conditions. But hey, then you discover all your bleeder screws are siezed and you've got leaking wheel cylinders or calipers - why'd that happen? Water collects at the lo...
How long has it been opened? You are correct the brake hydraulic system isn’t air tight. This is why overtime the fluid turns dark. The same with motor oil, it gets contaminated with moisture (water). The dangerous part of brake fluid being contaminated. In extreme case’s hea...
This info is not entirely correct. Water and hydraulic fluid cannot be compressed. You could use water in the braking system for a short time. The first issue is water will cause corrosion inside the metal components which you will not be able to see. The second issue is ...