Brake fluid after bottle is opened

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paulri

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Just bought some brake fluid. One or both manuals said to only use product that came from a new bottle, as moisture might accumulate inside an opened bottle. After filling up the fluid in both cars, I gave this away to a someone who works on his own cars. He told me that this wouldn't be a problem.

What do you guys do with opened bottles of brake fluid? Do you keep them and use the fluid next time? If so, how long do you keep them?
 
I try to do any open brake work on hot dry days, but you can't always plan that. I usually buy smaller bottles and use those. A big bottle with a lot of air left in it is not a good idea.

Interesting tester !
 
Hot dry days have more absolute humidity than late fall days. And absolute humidity is what counts in a sealed jug. Same with tires, I try to pump 'em up on crisp winter days.

I keep fluid in a half jug for up to a year or so. The MC reservoir is slightly vented, after all, so why not a jug? What goes in the car "looks" way better than what comes out.
 
By this logic brake fluid in your lines should be changed monthly. I open a bottle of brake fluid for 30 secs, pour it into the M/C. They are both subsequently closed. One sits on a shelf, one in the car. Both are in plastic containers.

There is no warning on the bottle to discard it. I'm sure 1 year would be fine.
 
Last edited:
Originally Posted By: Trav
Get a tester, if the humidity is low and the container is tight there is no reason to get rid of it but the only sure way of knowing is to test it.
This is also good for testing the fluid in the car to get a better idea of its condition.

There is one for DOT 3 and another for DOT 4.

http://www.amazon.com/gp/product/B00VX0L...rd_i=B005R3SSCM


A $26 tester is one way. I do need to keep in mind that I am talking about half a small bottle of brake fluid, so some $2.50 worth of fluid in a bottle. If I were to use it more often, or get bigger bottles, that would definitely pay off over the years.
 
Master cylinders are vented. I would worry more about the fluid in there than a bottle with the cap on.
 
I bought one of the electronic testers recently. My 2012 Mazda's fluid is still testing at less than 1% after 53,000 miles. I never thought about testing the fluid in an opened bottle. It tested at less than 1% as well and I opened it over two years ago.
 
This is getting to be kind of like doing a used oil analysis on oil, but here goes.

Is there a condemnation point for moisture in break fluid? How much would be too much?
 
Are you guys F#$ing kidding me? Seriously, you think you get a couple of ounces of air in the bottle that now its contaminated with moisture? Get on planet Earth. The amount of water you could get in there is like almost nothing. I am sure it could be calculated and it would be so small as to be insignificant. The cap on these bottle seals...WELL. And as another poster mentions, the reservoir is vented anyway. Honestly, the more I am on this site the more I realize the how rediculous it can get.
 
I use the leftovers of small bottle brake fluid to help clean my manual transmission clutch master cylinder reservoir. It's not a bad idea to feed and bleed in fresh fluid every 1-2 years or whenever the fluid starts to darken. I do that after every drive until the partial bottle is gone. You can't successfully feed and bleed a brake fluid reservoir...but you can for the clutch reservoir.

Fwiw, my 2002 Lincoln was likely on original brake fluid up until this year (65K miles). I had the local Monroe shop test it and they said it was still very good. IIRC they test for copper on their paper strips. I changed it myself a short time later. If you do open a bottle of brake fluid that you know you can't use right away, then get the remaining contents into a smaller bottle that's full to the top.
 
Originally Posted By: paulri
Is there a condemnation point for moisture in break fluid? How much would be too much?


The tester I bought indicates that when it reaches 3% it's time to change.
 
The critical level is about 5% water. Below that moisture level, there is essentially no corrosion. Above that level corrosion can be rapid.

I've frequently read the "never use an opened container" instruction, and it rubs me the wrong way. I suspect that it's a mix of CYA and mindlessly echoed advice. Unless you leave the container open or the cap loose, it's not going to absorb much water. There will be moisture that permeates through a plastic container, but that's true even with a sealed contained.
 
Originally Posted By: paulri
Originally Posted By: Trav
Get a tester, if the humidity is low and the container is tight there is no reason to get rid of it but the only sure way of knowing is to test it.
This is also good for testing the fluid in the car to get a better idea of its condition.

There is one for DOT 3 and another for DOT 4.

http://www.amazon.com/gp/product/B00VX0L...rd_i=B005R3SSCM


A $26 tester is one way. I do need to keep in mind that I am talking about half a small bottle of brake fluid, so some $2.50 worth of fluid in a bottle. If I were to use it more often, or get bigger bottles, that would definitely pay off over the years.


Some manufacturers recommend a 2 year change interval but that might not be applicable in the US where there are a lot of different climates.
In a low humidity climate once every 5-7 years might be fine, that's the point of testing it in the car.

It cost me more than $26 labor to properly change the fluid, a dark color is no real indicator as it could be just from the rubber components. 3% is a good exchange limit, over that internal corrosion can become a issue and the boiling point becomes significantly reduced.
A digital boiling point hot plate is slightly more accurate but that's overkill for most people and garages.
 
I remember when they went from steel to plastic bottles. No one wanted the plastic as it was permeable. Now all you can find is plastic for the most part.
 
Originally Posted By: ford46guy
By this logic brake fluid in your lines should be changed monthly. I open a bottle of brake fluid for 30 secs, pour it into the M/C. They are both subsequently closed. One sits on a shelf, one in the car. Both are in plastic containers.

There is no warning on the bottle to discard it. I'm sure 1 year would be fine.




This guy makes sense.

The cap on the bottle sure seems to seal better than the cap on the master cylinder reservoir, and the conditions for the bottle sitting on the shelf in the garage are better than the master cylinder sitting outside in the rain.
 
Based on the idea that you should change the fluid every couple of years. I will use an open container for top offs, but only new fluid for flushes.
 
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