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So what I've heard is that brake fluid should always be used new since it absorbs water. But I have an opened container of brake fluid that I used halfway to bleed brakes and have kept the cap on it sealed tight ever since. It's been sitting for about 8 months now. Can I use it or should I just spend the money to buy brand new never-opened brake fluid? Thanks
 

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Brake fluid is hydropillic. That means is will draw water to it, even in a half full bottle. I won't use brake fluid that has been open.
 

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It's like past due food and expired medication, up to your individual thinking, however brake fluid is cheap enough. On the other hand, dealers and repair shops regularly use opened containers all the time. Choice is yours.
 

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First to answer the OP's question, at 1/2 full and sealed for 8 months it's probably OK for "topping off" a reservoir--however for what even DOT 5.1 fluid costs I would not use it to flush/refill a system after (flush maybe if it were all run through followed by new fluid.


Now for some bored retired engineer with insomnia babble:

Glycol based brake fluids (DOT 3, 4 and 5.1) are hygroscopic, not hydrophilic.

Hydrophilic substances are those that can be dissolved in water through hydrogen bonding--and in the absence of insufficient water to form an aqueous solution do behave similarly to hygroscopic materials. Hydrophilic substances are dissolved by water because water disassociates it's structure, separating its negative and positive charged atoms (anions and cations).

However glycol based brake fluids are hygroscopic. I.e they are not dissolved by water but rather attract and trap/hold water. The water content is suspended between the molecules of the base material's structure, this is what reduces the boiling point of the fluid the more "wet" it becomes.

The simplest (and not at all chemically correct) analogy to illustrate the differences of hydrophilic and hygroscopic substances is to take a small block of sugar, and a similarly sized piece of sponge. Put each in a glass of water then after a few minutes remove each from its glass and attempt to squeeze out the water.

This cannot of course be done with the sugar block which have at least partially dissolved, however the water can be physically removed from the sponge and once completely dry it will return to its original size and shape.


Just as an FYI/FWIW, the dry and wet boiling points of DOT approved brake fluids are determined at 0.0% and 3.7% water content....
 

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I like your science lessons Cliffy. It's not boring!

Elkk, workshops keep fluid open only usually for a few hours or a day or so with the cap on tight so it's ok to use it. They know what is acceptable. The turn over of brake fluid in any workshop is very high and it's in daily use for topping up or bleeding. More modern Dot 5 fully syn is less hydroscopic anyway so it's not so much of an issue now. It needs changing less also.
 

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I like your science lessons Cliffy. It's not boring!

Elkk, workshops keep fluid open only usually for a few hours or a day or so with the cap on tight so it's ok to use it. They know what is acceptable. The turn over of brake fluid in any workshop is very high and it's in daily use for topping up or bleeding. More modern Dot 5 fully syn is less hydroscopic anyway so it's not so much of an issue now. It needs changing less also.
Thanks for the info, I understand. I've seen gallons of DOT 4 here being used until empty, months sometimes. Depends on the size/workload of the shop I guess.
 

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It's certainly a heavy toll on the environment to dispose polyethylene glycols opened more than just a couple of hours. Hermetic containers could be much better option.
 

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You can always use old brake fluid to strip paint! At least every drop I have had land on paint pealed it in a day or two.
 

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I'm a good conformist and always use a new can of brake fluid to even just add a little to top off the brake and clutch reservoir and then I throw the entire 12OZ can away:rolleyes:

I also have been programmed to follow the 3,000 mile oil change rule, even when I use $10 a quart synthetic:rolleyes:

I have been brainwashed to order cheese and bacon on my burgers and to always supersize, even though my whole body is already 'Supersized':rolleyes:

It's all a bunch of :pottytrain4:

No vehicle repair shop throws away their brake fluid because they worry about water absorption as this would not be economically feasible.:sleepy4:

All of these retailers/ wholesalers want us to be wasteful apparently to help their bottom line:D

***** If an opened and tightly closed can of brake fluid will absorb water what makes you think your clutch and brake reservoirs won't do the same?:confused: I have bought used bikes/ scooters that have NEVER had their brake fluid changed, in many, many years and the brakes were superb---weird or telling?????:rolleyes:

Sam:cool:
 

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You can always use old brake fluid to strip paint! At least every drop I have had land on paint pealed it in a day or two.
From my understanding only the glycol based break fluids will strip paint and absorb moister. Silicon based break fluids do not. Silicon based break fluids do not compress as good as glycol and will make your breaks a little mushy. That is why it is not used in race cars.
 

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Please don't break your brakes. ;)

:D
 

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"Now for some bored retired engineer with insomnia babble";
Thank you CliffyK for your knowledge, I have learned something.

We talk about risk in riding. There are also other types of risk. I stepped out and said something about brake fluid without the knowledge needed to back up my statement. I stand corrected.

Doug
 

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Thanks for the info, I understand. I've seen gallons of DOT 4 here being used until empty, months sometimes. Depends on the size/workload of the shop I guess.
Yes you are right elkk. Small workshop usually use a pump that displaces the air from the cans as they dispense the fluid so moisture cannot enter (easily). Or they use only small cans to distribute the fluid. Big workshops like the ones I worked in use as much as 40-50 litres a day when doing complete vehicle fluid changes and moisture absorption is not a problem especially when using synthetic DOT5 as that not hygroscopic. So no one need worry about that aspect. The guys know what they are doing.
 
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