Hi, Van,
The evaporation rate of ordinary, glycol-based fluid is considerably slower
than water...more like anti-freeze...so it might seem like it doesn't
evaporate, but I can assure you that glycol-based fluids DO evaporate. I've
seen evidence of this dozens of times. It's certainly not a rapid
evaporation rate, but it does evaporate. I've had brake-fluid soaked rags
completely dry out. I've seen dusty, brake-fluid-soaked backing plates dry
out. When I've spilled brake fluid on the rusty exteriors of master
cylinders, they dried out in anywhere from a few days to a few weeks,
depending on temperature and how much the vehicle was driven. I've even
heard that it is a faster evaporation rate than silicone fluids, but I have
yet to verify this by doing an experiment.
Even motor oil dries/evaporates, although it is at an extremely slow rate.
Synthetic oils seem to evaporate even more slowly.
When a glycol fluid evaporates, certain volatiles evaporate more rapidly
than others, so there may be some residual components of the fluid remaining
for quite a while. One clue to this rapid evaporation is the strong smell it
gives off.
By the way, I actually demonstrated this to someone not too long ago. Try
spreading a film of DOT 3, 4, or 5.1 brake fluid on a non-porous surface and
then wipe most of it off, leaving a very thin film. Come back in a couple of
days. It will be gone, unless you are near 100 percent humidity or the
temperature is extremely cold.
In any case, evaporation should not be an issue with either type of fluid in
a tight, leak-free system. Even on systems that do not have a rubber seal on
the top of the fluid reservoir (small breather opening open to the air) the
evaporation rate should be negligible.
That brings up a point, however. If you have a system that exposes the brake
fluid in the reservoir to the outside air, even if it is only through a
small breather opening, you should change glycol-based fluids quite
frequently, since they will absorb more moisture from the outside air than
they would in a totally sealed system. The percentage of water might go as
high as 4 or 5 percent, which can start to do some damage, especially in old
fluid in which the corrosion inhibitors have been depleted.
Regards,
Alan
(Salt Lake City, Utah)
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