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Discussion Starter #1
I have to top up my front brake reservoir with DOT 4 my question is after I top it up is that bottle of brake fluid garbage cuz it's been exposed to air? Can't I just put the cap on it and just store it for later use?


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Unless your going to use it like the next day.. might as well as toss it.

Can't store brake fluid, it turns all brown and icky kind of quick with air exposure.
 

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I helped my brother flush his brakes on his Toyota. He asked me if he should use up a can of fluid that he bought a year ago. If it wasn't for this site I would have said Hell Ya! I learned alot here and maybe some of it is Over Kill but it's better to be Safe than Sorry, especially when talking about Brakes. Yup, I told him to throw out the old fluid and in with the new.
 

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Discussion Starter #4
Damn that was an expensive top up....thanks for the info and by the way that's a seriously kick ass bike u have I would give just about anything for mine to look as nice as yours.


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I helped my brother flush his brakes on his Toyota. He asked me if he should use up a can of fluid that he bought a year ago. If it wasn't for this site I would have said Hell Ya! I learned alot here and maybe some of it is Over Kill but it's better to be Safe than Sorry, especially when talking about Brakes. Yup, I told him to throw out the old fluid and in with the new.
Ya I would if to him the same like you say better to be safe than sorry.


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Yes, of course you can save the brake fluid in a re-sealed can for later topping up.

When they say to use a new sealed can, they are talking about when you replace the brake fluid. You don't want to use a can that has been sitting around for years. They say brake fluid starts absorbing water within 15 minutes, so you should replace your reservoir cover and brake fluid can cap right away after you add brake fluid.

Think about it logically!
If the brake fluid in a re-sealed can is going to go bad right away, then the brake fluid in the brake reservoir will also go bad right away. And that doesn't happen. The recommended change period is every two years.

So, if you are going to change your brake fluid every two years, then buy a new can every two years.
Don't know about yours, but both my brake reservoirs have rubber sealing gaskets to seal out the air.

Off hand, I don't know of any brake fluid container that comes that way.
 

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Here's the thing, it's not air- but moisture. Yes, the reservoir has a vent, but it's designed to let as little moisture in as possible (it uses a membrane to accomplish this). Once exposed to moisture, the fluid begins to break down. Even in a tightly-closed bottle, that bottle of fluid will be exposed to much more moisture. It'll break down and become ineffective. Then that moisture laden (brake fluid is hydroscopic; that means it sucks moisture whenever it can- it's thirsty) fluid contaminants and ruins the rest of the fluid in the system. The result is spongy brakes, poor performance, and potential corrosion and break down of components in the calipers.

With moisture in the fluid, the boiling point is lower; leading to a potential for failure under hard braking. (In other words, someone pulls out in front of you, you grab those brakes with a vengeance, you begin to slow down; the moisture boils, suddenly your brakes stop working. That $6 bottle of brake fluid just got a lot cheaper didn't it?) Plus, of course, the long term effects of water in the brake fluid causing corrosion and other damage 'down the line'.

Brake fluid is an awful substance. If there was something practical and better we'd probably use it; but it has to stand up to a lot of heat and resist breaking down without actually flowing or being filtered (like engine oil). It needs to be able to compress yet be rigid, etc. The result, is that we use this rapidly breaking down fluid that is both hydroscopic (absorbs water with a vengeance) and ruined by moisture. And yes, it will pull moisture from the air -more than enough to ruin it- through a closed brake fluid container. Not to mention all of the moisture it sucked in when the cap was open (and so did the master cylinder when you had it open but you can only do what you can do).

On that though; OP, what was your reason for topping up and not replacing? On bikes, we have little brake fluid. It should be flushed every year or two. Brake fluid doesn't 'flow' so the fluid in the sight glass is not an indicator of the quality of the fluid where it counts (down at the caliper). Mine is usually still clear at the master cylinder but jet black at the caliper by a year.

Usually, brake fluid is only low for one of three reasons. 1) A leak. But that's probably not the case. Otherwise you'd have mentioned that! 2) Worn pads. The farther pads have to travel, the more fluid is needed to take up that space, so sometimes fluid is low when pads are worn. Might wanna double check them! Or 3) Fluid has broken down significantly.

I can tell you if the brake fluid in your master cylinder sight glass is dark; the fluid is long, long, long gone. And has been for quite a while.

There's almost no excuse for the kinds of brake problems and need for caliper rebuilds that some people have. If fluid is cared for, flushed regularly, and pads are replaced timely brake system components should be reliable and nearly maintenance free for years and years and years.
 

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Just occured to me, every time you go to Jiffy Lube and they check your brake fluid, they are screwing you over!
 

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Just occured to me, every time you go to Jiffy Lube and they check your brake fluid, they are screwing you over!
Dunno about Jiffy Lube but my brother in law used to manage a valvoline. At least there, the cap doesn't come off unless it's low (which you can see from the outside; there's a reason it's clear! The less the cap comes off the better). The fluid they use comes from sealed containers and is used in a day or two or discarded.

But I keep my fluid fresh and full so hopefully they never have a reason to open mine! lol.
 

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As usual Romans5.8 you make a lot of really good points. However, unless I missed something in your looong narrative, I did not see an explanation of how a resealed brake fluid can exposes brake fluid to moisture any more than a brake reservoir.

We all know that as brake fluid go bad it turns brown instead of clear. In my many years of working on brake systems on cars and motorcycles, I've yet to see a resealed brake fluid can pour out brown brake fluid. It's always been crystal clear.

If brake fluid was that unstable as you seem to suggest, then the manufacturer would be having us change the brake fluid every 30 days.

And I'm sure there are cars and bikes than haven't changed their brake fluid in 10 years (not recommended) and the brakes still work.
Well, it's not resealed. It's just reclosed. And that's the problem. Again, the fluid is hydroscopic. It's thirst. And yes, it's pulling moisture in from the air through the bottle, and cap in the fluid. Moisture can creep it's way in past the threads and everything. This stuff pulls water in like a vengeance.

It won't necessarily discolor without heat or a LOT of moisture. It can be clear, but beyond the 'spec' for the amount of moisture it can contain, even just a couple of days after being opened. While the membrane in the master cylinder has prevented this continual onslaught of moisture on your bike. Though even that can't stop it from breaking down due to heat and eventual moisture so it does need frequent replacing. Especially on smaller systems, like on our bikes.

And yes, lots of 10 year unmaintained brake systems out there. Eventually they'll have caliper failure or some other failure due to the moisture boiling, but sometimes you get lucky. It's no skin off of my nose how anyone maintains their bike, I just wanted to pass that information along. You might take a 3 year old bottle of fluid in to top off your 5 year old fluid in your system and be just fine. Or you might be the unlucky one to find that your brakes quit working altogether because the fluid has overheated due to the moisture. And that will happen when they are needed most; as they don't get all that hot cruising down the interstate!

My neighbor growing up had a Ford Taurus she bought brand new. Well, nobody ever told her you needed to change the oil (legitimately did not know). She would just occasionally top it off. 90-something thousand miles, original filter, topped up oil, the engine finally blew. So does that mean we should all extend our drain intervals to 85k? No, of course not. Just means she got lucky! And I can tell you that I definitely do not want to be gambling with the brake system.

For the record, a bike is a lot more sensitive than a car. For two reasons. 1) Much less fluid, thus it takes longer for it to be 'out of spec'. 2) Much more heat. Lots of cars can run DOT 3 (even though DOT 4 is about all you'll find anymore). DOT 3 and DOT 4 are essentially the same but differ in one important way; their maximum temperature. Our bikes REQUIRE DOT 4 because the brakes will get hotter than DOT 3's max during hard braking. Moisture lowers that max temperature. Therin lies the biggest reason (if you aren't worried about sponginess or the eventual brakedown of your brake components) for replacing fluid routinely and using the freshest fluid you can.

Discoloration doesn't mean everything. Engine oil can break down and remain clean looking. The discoloration of engine oil is from heat and combustion by products. Likewise glossy-black, foul smelling oil could potentially still be well within spec. Otherwise you're right, generally fluid color is an indicator of what's going on, but it's not the ONLY indicator; and things change in a different environment. Clear fluid in the system is generally an indicator it's A-OK. If it's been exposed to heat and pressure but isn't severely discolored, it's probably fine! Clear fluid in a bottle doesn't mean much of anything though.
 

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The term is hygroscopic.

New bottles of brake fluid have a thermally applied non permeable seal applied at the factory. The thread engagement between the bottle and the cap will not provide the same level of sealing.

For a standard user, an open bottle should be disposed of after a few months.

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Romans5.8, you make several claims such as the one above that just don't make sense to me. When people start making claims that moisture comes through the bottle, I'm totally baffled.
I don't know if you really believe what you are saying or you're being ridiculous because you like a heated argument.
If what you say is true then the bottles sitting in the auto store would be sucking up moisture.
Time for me to leave this ridiculous conversation!
LOL I think roman5.8 missed his calling and should have tried his hand as a Public Defender!! :D
 

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The term is hygroscopic.

New bottles of brake fluid have a thermally applied non permeable seal applied at the factory. The thread engagement between the bottle and the cap will not provide the same level of sealing.

For a standard user, an open bottle should be disposed of after a few months.

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Er. Yeah. Hygroscopic. That's what I meant :D


Sfairy it wasn't meant as a heated discussion. I apologize if I misread your intention of your posts and it was getting to that point. Was simply a matter of discussion. That's how we learn; we discuss. I may learn something from you, you may learn something from me, and if we're lucky, someone else will jump in and teach us something. Like the proper spelling of hygroscopic! Lol.

To answer your question though, it's through the threads. I suppose I worded that poorly. No the bottle doesn't "breathe", but the threads make a poor seal. The disphragm and small "membrane" (not sure what the term is so that's what I've been calling that) help reduce moisture that enters the system. While not a perfect seal, it's better than coarse plastic threads.

As far as the fluid color, I'll snap a pic next time. Maybe you can tell me what you think. But I flush the fluid every spring, about 15k miles, and usually the first pump or two is black, subsequent pumps are that icky brown color, and eventually clear. (I use the old pump-open-close-release method)
 

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The term is hygroscopic.

New bottles of brake fluid have a thermally applied non permeable seal applied at the factory. The thread engagement between the bottle and the cap will not provide the same level of sealing.

For a standard user, an open bottle should be disposed of after a few months.

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I agree. There is a balance here. To toss out a bottle of brake fluid simply for opening it and having the cover off for a minute or two is crazy. Also, the threads never have been part of the seal, they just hold the cap on. There is a gasket, for lack of better terms, inside the cap. No, not as good as the original seal, but certainly it doesn't allow air freely into and out of the bottle.

On the other hand, if you need to top off your brake fluid repeatedly perhaps one should consider fixing the problem, then storage of brake fluid becomes a non issue.

Not that i dont know about the hygroscopic properties, i just feel the implications thereof in this thread have been a bit strong at times.
 

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How the brake fluid in your bike which is subject to heat, stress, rain, dirt from the outside environment can last 2 years,
but, according to this forum, the brake fluid in a air tight clean container which not subject to heat, stress, rain, dirt from the outside environment can't last as long, just does not make sense to me.

What's the expression?
It just does not pass the smell test!
*shrugs* Maybe not! I just know what I know. Like others, I don't reuse brake fluid. Within a few days? Sure. But if it's been sitting on the shelf for months I'll spend the couple of bucks and get another bottle. I really didn't think the discussion was going to hit so close to home for some folks. Figured we were just discussing something. Whoops!

Whoaru made a great point though. If you NEED it within a few months on the same vehicle, something needs FIXED! Either way it should be looked at, usually low fluid is indicative of an issue (most commonly, worn pads)
 

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I looked at the instructions on Dot3 and a Dot4 brake fluid bottles that I have and here is what they say.

"Store brake fluid only in its original container. Keep container clean and tightly closed to prevent absorption of moisture"
Mine says "Keep refrigerated after opening" so I have it next to mustard and ketchup. :D
 

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ya know the whole got to use it right away or throw it out is probably nothing more than an old wives tale.... kind of like once you switch an engine to synthetic oil you cant go back to conventional.....
 
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