From: Jim Newton (jnewton@laurel.com)
Date: Wed Jul 10 2002 - 20:37:52 PDT
Hi List...
>>Why would the brake fluid ONLY absorb moisture if the vehicle is not
>>driven daily?
>The same reason water accumulates in engine oil if not used.
>Condensation. When you use a brake system you are heating it up. Plus
>the seals are being used vs just sitting static. This means they
>won't be drying out as much and as such won't be having as many
>problems from non use.
OK, not to nit pick, but...
But it isn't condensation that causes non-DOT5 brake fluid to absorb
moisture from the air...it is the hygroscopic nature of the fluid
constituents themselves. Non-DOT5 fluid wants to suck in water SO
BAD! Moisture is drawn in by the massive hygroscopic pressure of the
fluid right through the pores of the rubber hoses, seals, and in the
vent hole of the reservoir. Even if the brake system was completely
sealed, the rubber and non-metal components would allow the tiny
3-atom water molecules to seep through the complex porous rubber
molecule mesh with relative ease. That's why latex balloons go flat
in a day or two when filled with air or helium...the gas molecules
slip out through the rubber molecule structure. Mylar balloons stay
inflated longer because the metal used in mylar does not contain such
coarse pores.
Besides...the brake components would not condense moisture from the
air unless the lines and components were cold and the air was warm and
humid...and even in that case, the moisture would not condense INSIDE
the brake hydraulic circuit except maybe in the reservoir, but that
would be quite unlikely if there was any fluid in the reservoir.
And it isn't heat from braking that drives moisture out of brake
fluid...the moisture stays in the fluid once it is absorbed.
So I'm still curious why the consensus in this group seems to be that
brake fluid that sits around idle is more likely to absorb water.
I think it is just junk science voodoo witchcraft hearsay. Has anyone
done the experiment? Completely flush and change the brake fluid in
your (non-DOT5) vehicle with fresh non-DOT5 fluid. Then open a fresh
container of non-DOT5 fluid and replace the cap, but poke a hole in
the cap exactly the same size as the vent hole in your brake fluid
reservoir. Drive your vehicle a lot every day, and be sure to brake a
lot to boil off the moisture. Meanwhile, store the opened and sealed
container of brake fluid in your barn or outdoors where it will not
have rain or dew dripped into the "vent hole" you made (make a tent
out of foil so the air can reach the vent hole freely). In 3 months,
sample and measure the boiling point temperatures of the fluid in your
vehicle and the fluid in the container (or have the samples analyzed
by a brake shop). I bet they'll have exactly the same moisture
content...probably about 5%.
I agree that the seals will last longer if exercised.
I also agree that the engine of a vehicle that is stored for long
periods without being started will collect moisture in the crankcase
because the block will get cold at night and then when the day air
warms, it will condense moisture inside and outside the block. This
will happen in all engines, but if you run the engine then the
moisture emulsifies into the oil instead of pooling at the bottom of
the crankcase.
Those WHACKY physics!
--Jim "Ike" Newton
o 1984 M1007 CUCV Military Suburban 6.2 Liter (378 CID) Turbo-Diesel Engine 5/4 Ton Cargo Capacity, 4WD
o 1971 M35A2 Military Troop/Cargo Truck "Deuce and a Half" 478 CID Turbo-Diesel Multi-Fuel Engine Air Shift Front Axle 2 1/2 Ton Cargo Capacity, 6WD
See them at: http://www.CUCV.net
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