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Old 09-18-2017, 01:12 PM   #1
adouglas
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Default Random tech question re brake fluid

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. Small volume, little if any moisture.

After all, the bottle has air in it anyway when it comes from the manufacturer, right?

So how can it get contaminated if it's re-sealed after use?
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Old 09-18-2017, 01:44 PM   #2
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Possibilities:

The cap is NOT air tight and as such additional moisture gets in over time
The cap IS air tight but temp changes in the area it's stored affect the cap differently than the bottle allowing air exchange which allows moisture in
CT is so fucking humid that the little volume of air that replaces the poured out fluid is 90% water
There is no contamination beyond what the stuff you just poured into the bike was exposed to and the remaining fluid is safe to use as any time.

Those are the possibilities that I see. As for which one is the most likely? No idea. I usually just get the smallest bottle I can for the job I'm doing and toss the rest when I'm done.
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Old 09-18-2017, 01:53 PM   #3
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Want to be even more paranoid? Unless you're using a metal container, water can permeate the bottle. So even an unopened bottle with some age can be suspect.
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Old 09-18-2017, 03:19 PM   #4
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It can't, don't worry about it. Just make sure you dont use the wrong oil
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Old 09-18-2017, 03:30 PM   #5
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Ooh, goody! An oil thread!

Can we debate octane and ethanol while we're at it?
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Old 09-18-2017, 03:47 PM   #6
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Heh, no debates, DOT 3, 4, 5.1 all attract water. Plastic containers are slightly permeable to water, metal containers are not. Once you crack the seal, the clock goes from slow to fast on the condition of your fluid.

Now, the fun question is - At what point does it become an issue? You can get testers that'll measure the water content of brake fluid. You can get pocket testers for $15 to $30 if you want to be scientific about it, I just assume a season is long enough for fluid in a bike to need a change, and as you noted fluid is cheap so as soon as I can see any color change I toss the bottle. I've never had a system boil over so, I'm on the correct side of the maint schedule. I may be in severe overkill territory, might be right on the line, who knows?

(My cars get ignored, I don't wanna mess with flushing and bleeding an ABS system on a hybrid, I don't want that pain.)
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Old 09-18-2017, 04:14 PM   #7
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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?
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Old 09-18-2017, 05:04 PM   #8
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I've always thought the color change was due to wear from heating and cooling.

I also always just buy the smallest bottle when needed. Every year I used to chuck it. Between cars/motorcycles and toys I've never really cared or had it sitting around long term.
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Old 09-19-2017, 05:09 AM   #9
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After having my brakes fail because of fluid at a track day once in thompson I care haha. If it's going in a bike I always buy a new container.
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Old 09-19-2017, 05:21 AM   #10
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How do you dispose of old brake fluid? Because no mater how small a bottle I buy, I usually have enough to do two or three full flushes with it. My dump won't take it, and the local auto parts store is always "full".
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