yamahaSHO

In that respect, what you pour into your reservoir is just as much of a risk. It was exposed to open, moist air and the plastic does not seal off air/water nearly as good as a metal can.

dwight

I agree with _brandon's line of thinking. Each time you open and close the bottle, you introduce new air into the container. If you use 1/3 of a 12 oz container or brake fluid, you now have 4 oz extra headspace. If you open it again and use another 4 oz, you now have 8 oz headspace, and over the time the brake fluid has been exposed to 12 oz of air. By contrast the reservoir fill line is fairly close to the top of the container, so it doesn't have much room for air. More importantly, there's nothing you can do about that to reduce. Also, as stated, brake fluid is cheap. Why risk it?

On the other hand, Billman250 says it's fine, and I trust his experience more than academic speculation, so I'll unclench my sphincter and be a little less anal about this in the future.

SpitfireS

How much water would the air in the container hold?
Not that much.
It depends on the temperature and relative humidity of that air at the time of closing the container.
The brake fluid in the container can not (assuming the containier is sealed, and a steel container is pretty much sealed) absorb more water than is in there, IOW it dries out the air in the container.
After, lets say, 6 months, the condition of that fluid is way better than the fluid in you calipers.

Take your opened bottle of brake fluid to your local tire shop and ask them to flush it with N2 (if they have it)
That will reduce the water in the container to pretty much zero.
Good as new.
Even compressed air will do, as long as there is a dryer in that system.

redlineracing

Not sure why anyone would add color to BF.....to look for leaks maybe? Off the shelf stuff like Prestone Dot4 & Valvoline Dot4 is about the same color as ginger ale.

With this color ( for me) it's EZ to spot when a change is needed.

\rlr
carolina

dwight

Despite it's likely rhetorical intent, that's a good question.
I have two containers of ATE Super Blue. One virgin container, and one container that I opened and used to flush my brakes.
New - 1.230 kg
Previously Opened - 0.614 kg

According to the MSDS, the density is approximately 1.08 g/ml.

There fore I have used 0.616 kg, or 570 ml. So, there is at least that much air inside. When I shake the new can, there's a noticeable amount of space. I'll estimate that at being about 3 ounces / 100 ml.

So lets say there is 650 ml of free space in the can. And through the repeated pouring actions the air in the can is now similar to the air in environment.

I don't have weather data from when I used the can. Let's pretend it was 86 F (30 C), and 65% relative humidity. This gives us a saturated vapor density of 30.4 g/m[sup]3[/sup].

So, 650 ml * 30.4 g/m[sup]3[/sup] = 19.76 mg = 0.02 ml. This gives us 0.005% moisture content by volume.

If the situation could be worse. We had about a 1:1 ratio of brake fluid to air. That could be easily be 3:1. If it was 100 F, it air would hold 50% more vapor, and the humidity could also be higher, like 85%. Even with all this, it only brings it up to 0.03%.

The wet boiling point is typically defined at 3.7% water by volume. So, that's more than 100 times the worst case scenario. So the qualitative answer to your question is very little water.

So, back to my previous position. Brake fluid is cheap, so it's not a big deal to discard partial containers, but as Billman250 says, it's perfect safe to use them, provided you re-close them and don't store them open.

And if you want a little personal victory from a previous discussion, I pushed the caliper pistons back in when I was bleeding the brakes on my father's car yesterday. (But I was also changing his brake pads.)

SpitfireS

I can't help but detect some slight condescending content here, I hope that's because I'm not a native English speaker, also, I hope I'm wrong.

Not really, do as you please, it's your (father's) car.
Use the procedure because you agree it is a good one, not because "some guy on the internet" says you should.
That part ruined what you suggested as a victory anyway as you have to push them back to fit the new pads, maybe not ALL the way but still.....


Your calculations showed the absorbed water in the given situation minimal at its worst.

Next time give the ATE SL.6 fluid a try.
It is thin, much thinner than most and will make ABS act - noticeably - quicker, especially when it gets cold.
It works well in the clutch too allthough it may make the OEM clutch feel very light and smooth to operate, maybe too light.

vtec9

Rotating between Super Blue and an amber fluid each change makes brake bleeding pretty foolproof. When you see only the new color coming out of the bleeder, you know you've replaced all the old fluid in that line and have bled enough.