Brake fluid changes in general

I am curious to ask about the issue of brake fluid changes. I have just replaced the fourth brake line on my 1995 truck (150000 miles). These lines are heavily rusted on the outside. Has anyone cut open such a line to see what the interior looks like? That is, what evidence says that the problem is the hygroscopic brake fluid?

The brake lines fail due to the rust on the outside.

Fuel lines do the same thing.


The INside of the calipers, wheel cylinders and master cylinders is the proof of the hygroscopic nature of brake fluid. When the caliper piston is seized… the crud it is seized upon is rust or aluminum oxides. When the internal screw-type parking brake locks up solid, that is the effects of water in the brake fluid. When the aluminum caliper or master is pitted and the seals no longer seal, that is the effect of water in the brake fluid.

The steel brake lines are zinc coated. The zinc outside corrodes away in the environment and the bare steel is left to rust. The zinc on the inside is enough to keep the “wet” brake fluid from corroding the lines… hence @Tester’s comment about rust on the OUTside rather than on the INside.

The hygroscopic fluid causes other problems. It doesn’t cause the outside of the brake hardlines to rust. As the other two said, it causes things to rust on the inside of the system. Sometimes if you live somewhere that uses a lot of road salt, that internal rust isn’t as fast as the external rust is, so you end up replacing a brake line before you have to replace stuff that failed from internal rust. But if you aren’t getting regular fluid changes, you will end up having problems from that regardless of what happens to the hard lines. And not just rust.

Brake fluid with water in it is not as effective as brake fluid without water in it. When you get water in your brake fluid, its ability to continue working properly as it heats up degrades, because the water lowers the boiling point of the fluid. This means your brakes will get mushy from repeated applications or from very hard stops faster than they otherwise would have.

If you’re just tooling around Somewhere, Iowa at 30mph, you’ll probably never notice. But if you’re making a descent on a curving mountain road, you’ll notice in a sudden and frightening way, as your pedal will suddenly sink to the floor once the brake fluid heats up beyond its ability to cope.

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First you’ll SAY it… Then you’ll DO it!

If you survive it, you’ll need upholstery cleaner for your driver’s seat! (Just throw the pants away!)


I see some people recommending fluid changes every 30K. Seems excessive. Question: does the hygroscopic moisture cause the darkening of the fluid. Is that a good measure of when to change it? (I had a seized caliper on my 1996 Miata recently, so I am listening to the good advice here.)

you can buy test devices or test strips to let you know if fluid is good or bad

Change it every 3 years, not on mileage. It absorbs water over time. You can buy a fairly cheap tool on Amazon and other sites to test how much water is in it.

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Since the advent of ABS, it’s even more important to change the brake fluid every 30,000 miles

ABS pumps are not cheap!

A good measure as to when to change the brake fluid is measure the voltage of the brake fluid with a multimeter.

Set the scale to 2 VDC.

Place the positive lead in the brake fluid in the reservoir, and negative lead on the negative post on the battery.

If the reading is 0.3 volts or more, there’s excess moisture in the brake fluid.


I like the voltmeter test. I’ll give it a try.

that “depends…”

6 years from from the factory and 67K miles (from the first owner) was enough to get one rear caliper “semi-seized” on a used car my daughter bought some time ago - it was dragging and wore pads on one side

fluid was quite dark around the caliper, but was OK-ish in the master cylinder/tank, so if I went by the visual test I would not catch it

I discovered that since replacing all critical fluids is the first thing I do on any “new to me” / used cars

I replace all brake fluid every 3 years, regardless of mileage.

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