There are test strips you can buy to test the copper content of your brake fluid. And some also test the PH of antifreeze and boiling point and or freezing point. (Though one of the best most accurate way to test freeze point is to put a sample in a freezer). Amazon sells the test strips. if you get the ones with antifreeze test on one side, and brake fluid test on the other, do the brake fluid test first so you do not get any antifreeze into the brake fluid. And wear clean dry gloves so you do not touch the active parts of the strip. And there are pen size electronic test equipment that you can also buy on Amazon that will test brake fluid for how much water is in it, and they are not very expensive. ( Mine takes one AAA battery to power it, do not use Duracell, they leak can ruin stuff ).
One of the very important things to test brake fluid for is copper contamination.
Here are a few facts I learned working on cars, and in college chemistry II, and industrial engineering:
Because ABS systems use a machined block of aluminum with many small passages to provide the proper flow and pressure of brake fluid to all 4 wheels, and because copper and aluminum are on opposite ends of the electro-negativity scale, mix these two metals together with an electrolyte (such as impure water) and you can make a great battery, and or cause a lot of corrosion.
Nail copper gutters up with aluminum roofing nails and add in rain and moisture and you can watch those gutters fall down in a couple of years.
Plum an aluminum injection molds cooling passages with copper pipe fitting and you can kiss goodbye to the cooling passages in a couple of years, (can you say scrap a $20,000 injection mold).
Unfortunately the metal alloys of brake lines are made with some copper in the mix so the lines can be easily bent without breaking them, and also so they are not brittle and break with the vibrations they see in their useful life. But the down-side of having copper in the metal of those lines is that over time a small amount of copper will get into the brake fluid and find its way into the fluid that is in the ABS system. And if a small amount of impure water is also present then the aluminum of the ABS system will corrode, and because many of those passages are small, and valves require clean metal surfaces without corrosion, when that gets bad enough the ABS system will fail. And fixing that ain't cheap.
Now while it is true that some climates like Arizona have an average humidity that is low, another thing has to be take into consideration, in hot climates is an engineering rule of thumb ( rough calculation to get you into the ball park of how thighs will interact, not exact science numbers nailing it down to exact time required for something to happen ) is that for every increase or decrease of 10 degree C (or 18 degree F, which is the same amount of temperature difference) a chemical reaction will respectively happen twice as fast or half as fast. In other words use a car battery where the average ambient temperature is 10 degree C hotter than another area and that battery can be expected to wear out twice as fast. Which explains why car batteries do not last long in Arizona.
I have no hard data to prove it, but it is possible that brake fluid at an average year round temperature that is hotter than other areas will have copper leach into it faster. So, while the total water content may not increase in Arizona as fast as cooler areas, it is probable that copper content would increase faster. Proper test strips that are not out of date (can you say do not store them in a hot garage) will show you the level of copper in you brake fluid, and you should be testing that if you are not replacing your brake fluid regularly.
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Now as to boiling brake fluid:
The boiling temperature of brake fluid decreases as water content increases. This is a bad thing. When brake fluid boils it forms steam. And steam is a compressible gas (not a good thing to have in a hydraulic brake system, to say the least. Can you say I have absolutely no brake at all?) Brakes get very hot, and throw in hot weather and or long steep hills and they even get hotter. When steam is created it will expand and push the brake fluid back into the master cylinder when the brake is released and the steam is not under severe pressure. The next time you hit the brake you will be compressing the steam and not moving the brake caliber pistons and you will have NO Brakes. One of my brothers was using a rental truck to move a friend across country, and when crossing the Rocky-Mountains they decided to pull into a rest stop while going down a mountain. When he got back in the truck after the break the truck had no brakes at all. How often do rental companies change brake fluid, if ever? And it did turn out that the reason for the brake failure was that the brake fluid had boiled because it had too much water in it, and it does not take much at all. Anyhow, he was luck, because the truck was still in the level parking lot and the truck was not moving fast. But it could have been a lot worse.
Boiling brake fluid is no joke. It really can and does happen if the fluid has too much water in it.
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Now that all of this is said, if you want to put some really great brake fluid in your vehicles check out Bosch ES16 brake fluid and what is said about it on the Bosch web sites and YouTube. Great stuff that maintains good performance loner than others and prevents problems even when some contaminants are in it at a level that would cause problems with other brake fluid (though even this much better fluid has its limits about how much contamination it can tolerate and still perform well). And also its useful life is longer than standard brake fluid. I am not affiliated with Bosch in any way, but when I flushed the brake fluid in my 2016 Honda CR-V EX last year that is what went in, and what I plan to put in it in the future.
There are special applications where some requirements may exceed what Bosch ES16 can provide, and in those applications it would be wise to use something else better suited for that reason. But as for fluid for a daily driver this stuff is hard to beat in performance, though it is much more expensive that some three dollar can at Walmart or the dollar-store.