Random tech quesion re brake fluid

[adouglas]

Okay, so we're supposed to only use a freshly opened bottle of brake fluid when bleeding brakes, because the stuff is hygroscopic and once opened, will absorb water.

Fair enough, and it certainly makes sense to be conservative. Brake fluid doesn't cost that much, just do what they tell you, yeah, yeah, I get it... and that's what I do.


But in the interest of knowledge, riddle me this, Batman:

Say you open a fresh bottle, use half and re-cap it. Since the cap is airtight, the only air that fluid can possibly be exposed to is what's in the bottle, right? Small volume, little if any moisture.

So how can it get contaminated?

Discuss.

[dcj13]

I use brake fluid from partially full bottles and share your belief about the fluid's ability to absorb only the amount of water vapor present in the bottle.

[DannoXYZ]

Yep me too. I have noticed however, that metal tins tend to seal better and fluid lasts longer. Plastic containers seem to allow some extra moisture in.

[Flying]

I don't think that the problem is necessarily moisture in the air finding its way into the container on its own.

Brake fluid seems to be remarkably good at creeping through tight spaces via capillary action. Not too long ago I found a couple of containers (1 metal & 1 plastic) that were weeping brake fluid despite being capped firmly. I don't exactly know what caused the fluid to be drawn out, maybe it was my climate, but I wouldn't doubt it provided an interface for moisture in the air to find its way into the container.

[jkv45]

Drop the bottle into a Ziploc bag and squeeze the air out for extra security maybe?

I also use brake fluid from a closed, but non-full, container. Unlike the fluid in the brake reservoir, it's never dark or discolored.

Interesting how the fluid in a sealed brake reservoir gets dark, and I assume, contaminated over time.

[adouglas]

Related question:

We all know that as the fluid in your brake system ages it gets darker, going from pale yellow to amber to -- if you're really lazy -- the color of Jack Daniels.

Is it water absorption that does that? Oxidation? UV exposure through the translucent reservoir? Heat? Something else?

Would color be a way of judging if opened/resealed fluid is still good?

[DannoXYZ]

New fluid should be clear with slight yellow tint.

What darkens fluid in use is rubber-seals in master, slave and brake-pistons. Some of it is microscopic pieces of rubber wearing off. Another part is black rubber dye itself getting leeched out by brake-fluid.

Colour is independent of functionality and similar looking fluids may be 100% fine, or it may be 10% depending upon how much water has been absorbed.

[Singh2jz]

What about thread tape for an even tighter seal???

[adouglas]

Originally Posted by Singh2jz

What about thread tape for an even tighter seal???

Question still remains.

Given that a plastic bottle with a snug screw cap can be squeezed without leaking air, it's safe IMHO to assume that it is, for all intents and purposes, airtight.

The whole "use only fluid from a freshly opened bottle" has the feel of conventional wisdom that, through repetition, grows beyond its original intent. Not unlike the MSF admonition to never brake while leaned over in a corner. Rooted in logic but distorted into an absolute that is ultimately misleading. In the case of fluid, I suspect it may originally have been "don't leave your fluid exposed to the air, because it'll eventually go bad."

Still can't see a reason why the fluid in a tightly capped bottle would go bad, unless there's some mechanism I'm missing.

PS: I have a half-full, sealed bottle of Motul RBF600 that's been lying on its side for a good long while, and it's completely dry on the outside. No capillary action creep....

[jkv45]

Originally Posted by adouglas

Question still remains.

Given that a plastic bottle with a snug screw cap can be squeezed without leaking air, it's safe IMHO to assume that it is, for all intents and purposes, airtight.

The whole "use only fluid from a freshly opened bottle" has the feel of conventional wisdom that, through repetition, grows beyond its original intent. Not unlike the MSF admonition to never brake while leaned over in a corner. Rooted in logic but distorted into an absolute that is ultimately misleading. In the case of fluid, I suspect it may originally have been "don't leave your fluid exposed to the air, because it'll eventually go bad."

Still can't see a reason why the fluid in a tightly capped bottle would go bad, unless there's some mechanism I'm missing.

PS: I have a half-full, sealed bottle of Motul RBF600 that's been lying on its side for a good long while, and it's completely dry on the outside. No capillary action creep....

It's possible it was much more of an issue back-in-the-day before synthetic brake fluid became readily available.

One advantage of synthetics is they have a closed molecular structure that doesn't allow free 02 atoms to attach and "oxidize" like conventional structures do.

[Singh2jz]

Originally Posted by adouglas

PS: I have a half-full, sealed bottle of Motul RBF600 that's been lying on its side for a good long while, and it's completely dry on the outside. No capillary action creep....

I'm gonna have a half bottle of motul remaining too, after I work on the brakes lol. So I'd love to be able to save the rest for next time. So I guess we gotta wait for some more responses to see wassap.

EDIT: I agree with Jay..

[dz0]

Any one that wants the throw a half bottle of brake fluid away simply because it was opened and some content was withdrawn, please send it to me. I'll use it.

I wouldn't worry about it. If you notice your brake performance diminishes at the track after using the second half of the bottle a year or two later - then don't do it again. If you are riding street in a relatively safe manner you would never even notice it.

[redonninrf]

I've wondered the same thing.

I'm not familiar with bike master fluid reservoirs, but the ones on cars don't seem to seal any better than a plastic or metal can would. So in a similar vein: Why is it ok to leave the fluid in a street vehicle for a couple years if we're not supposed to use 2-month old leftover fluid?

[Ram Jet]

Originally Posted by redonninrf

I've wondered the same thing.

I'm not familiar with bike master fluid reservoirs, but the ones on cars don't seem to seal any better than a plastic or metal can would. So in a similar vein: Why is it ok to leave the fluid in a street vehicle for a couple years if we're not supposed to use 2-month old leftover fluid?

I have read that brake fluid can absorb water right through the rubber brake lines. I really don't believe this is true. Myself, I bleed out the old brake fluid every spring and will not use a previously opened can of brake fluid. Really guys, we are talking about a $5.00 can of brake fluid. I have another use for a a partial can of brake fluid anyway.

By the way, it's not OK to leave old brake fluid in your auto either. I change my car's brake fluid every three to five years. Who cars about cars anyway.

Bill

[dz0]

A penny saved is a penny earned.

[csmith12]

Haven't noticed any issues with using a partial container of brake fluid if sealed well and not contaminated. The fluid in my track r6 is poured from a gallon jug that is 2.5yrs old. And I trust it to slow/stop me from 145mph at least 40 track days per year. Although, it does get flushed every spring w/ old NAPA DOT 4.

I will NOT buy a gallon jug again though, Imma stick with smaller containers for the bikes. It's just too much of a hassle for me to ensure it was sealed properly. And always guessing if someone used some and didn't seal it back well.

[adouglas]

Per the OP, the question isn't about whether or not to use brake fluid from a previously sealed container.

It's about whether, and if so how, brake fluid can degrade/get contaminated if the container is resealed after it's initially opened. A reality check on conventional wisdom.

[corksil]

Originally Posted by adouglas

Per the OP, the question isn't about whether or not to use brake fluid from a previously sealed container.

It's about whether, and if so how, brake fluid can degrade/get contaminated if the container is resealed after it's initially opened. A reality check on conventional wisdom.

Absorbs water through the container holding it. A chemical reaction of absorption that accelerates once it has begun. The brake fluid is considered stable until the container has been opened to the atmosphere. This starts the reaction, and it's all down hill from there.

[adouglas]

So the LDPE that plastic bottles are made of is water- and air-permeable?

Hmm....

[csmith12]

Originally Posted by adouglas

So the LDPE that plastic bottles are made of is water- and air-permeable?

Hmm....

Yes... he is correct. not all LDPE is waterproof, some are just water resistant and the sealer, aka the "lid" just adds to the problem. In this case, you get what you pay for.

https://www.upcinc.com/resources/materials/LDPE.html

Stress cracking is the biggie here, and for many of us... is the flux in temps that cause leakage and absorption of contaminants.

[csmith12]

I have been doing some very high level mathematics on materials, in specific rubber, but haven't limited it to that. Plastics have come into the equation as well. Here is the common sense real deal...

From dust you were born and to dust you will return.

If you truly believe that just about any material can 100% protect from the outside biodegradable elements, you are 100% mistaken. Given enough time and contact with outside forces, your brake fluid will be "crap."




What does that really mean? You're good storing it in a complete "sh*t" bottle for a season or two, unless you live in the tropics and leave the cap off for a year. lol Or.... you have an application where the temps of the fluid is borderline usage. aka... upgrade from dot 4 to dot 5.1 unless feel becomes an issue. Ps.... don't bury your brake fluids in the dirt over time. hahahahahahahahahahah

[csmith12]

Sorry ya'lls, I know this type of post is not really "me." My paw, that works for NASA has been visiting and hence raised the bar. lol

You should hear dinner conversation about "quantum mechanics" as it pertains to the perception of reality. It's scary honestly....

[DannoXYZ]

Originally Posted by csmith12

Yes... he is correct. not all LDPE is waterproof, some are just water resistant and the sealer, aka the "lid" just adds to the problem. In this case, you get what you pay for.

https://www.upcinc.com/resources/materials/LDPE.html

Stress cracking is the biggie here, and for many of us... is the flux in temps that cause leakage and absorption of contaminants.

Originally Posted by csmith12

I have been doing some very high level mathematics on materials, in specific rubber, but haven't limited it to that. Plastics have come into the equation as well. Here is the common sense real deal...



If you truly believe that just about any material can 100% protect from the outside biodegradable elements, you are 100% mistaken. Given enough time and contact with outside forces, your brake fluid will be "crap."


...<img>...

What does that really mean? You're good storing it in a complete "sh*t" bottle for a season or two, unless you live in the tropics and leave the cap off for a year. lol Or.... you have an application where the temps of the fluid is borderline usage. aka... upgrade from dot 4 to dot 5.1 unless feel becomes an issue. Ps.... don't bury your brake fluids in the dirt over time. hahahahahahahahahahah

Originally Posted by csmith12

Sorry ya'lls, I know this type of post is not really "me." My paw, that works for NASA has been visiting and hence raised the bar. lol

You should hear dinner conversation about "quantum mechanics" as it pertains to the perception of reality. It's scary honestly....

Great data and real quantitative analysis! It's great to get fresh perspective rather than re-hashing of the same old-wives-tales without any supporting data. Kinda like old adage of not running down your tank because it'll suck up crud from bottom...

I've always bought brake-fluid in metal tins for this very reason...

[jkv45]

So the bottle itself can let moisture in? How about putting the bottle in a Ziploc bag with the air squeezed out?

Or is that permeable too?

I think we are splitting hairs at this point.

[csmith12]

Splitting hairs, of course not but I feel where you are coming from.

If one could vacuum pack the fluid storage bottle and/or enclose it in another layer of material with all air removed (put it in a bag), one could extend the life of the fluid by many, many years of storage. It makes logical sense that adding layers lessened the effects.

But common... who is going to go through all that when you can just "care less" vs the < $5 cost of a new bottle. Hence, my original comment to the post.

It's just too much of a hassle for me to ensure it was sealed properly.

[dz0]

My impression is that when you open the bottle and dump some out, that missing liquid is replaced by the current air - which contains moisture. The moisture in that air migrates into the remaining fluid. Again - please send me your open unfinished bottles because the amount of moisture is generally trivial. The idea that the bottle of liquid is sucking water out of the air around it (in the timespan of a year or two) is borderline preposterous. Open the bottle every three days and I'm sure it will pick up much more moisture than if it sits sealed for a season or two. If you are in a 100% humidity environment it's going to pick up more moisture than if you are in 15% humidity.

The second thing someone mentioned is contamination. Use the bottle with greasy/dirty hands and leave the cap off and you increase the chance of getting junk in there. Smoke a cigarette over the bottle and drop an ash in there. A fly thinks it's water and lands in the bottle. A lot of things can contaminate the bottle. You use the bottle instead of a container to bleed the brakes and the line gets dirt in there, or backfeeds old fluid in.

As a general rule - it's OK to keep the remainder and reuse the stuff. You won't have massive brake failure as a result. Consider the # of times the bottle was opened, the environment in which is was opened, and the age of the fluid. After a pile of openings (if you get that far before emptying it) or a good number of years - pickup a fresh bottle.

The study cited above is for a canister (of a SPECIFIC composition that is not necessarily the same as what brake fluid is kept in) that is immersed in 100% water. I recommend against storing your brake fluid in the pool.