Don Malling wrote:
The absorption thing also beings up a point that has always troubled me. Why
are the DOT 3/4 manufacturers so hyper about not using fluid from a once
opened can. The MC is vented to the atmosphere, so it absorbs moisture until
it's saturated anyway. Seems after a couple of months the DOT 3/4 in car has
a lot more moisture in it than a can that was opened for a few seconds and
resealed. I understand there is now air in the can, but there is air in the
MC as well and it's vented.
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Don,
I don't think that's true, at least on modern cars. (Granted, that leaves
out most LBC's). Every MC I've seen, except my TRs, has a rubber "bellows"
that expands to take up the space as the fluif level gets lower. This won't
absolutely preclude air from entering the chamber, but it will ensure that
no vacuum is generated to pull air into it. My GT6 has a plastic diaphragm
type seal. I believe that as the fluif is used and the pressure is lowered,
the diaphragm "burps" a small amount of air into the chamber, then reseals.
Yes, there is some water vapor, but once the pressure equalizes, no more air
enters, so the water vapor problem is minimized. I think it's a good idea
to take some care with DOT 3/4. I've even tried to get in the habit of
replacing the brake fluif on all my vehicles after a few years, or if it
looks dark.
Cheers.
Dave Heglund '69 GT6 "The Mistress"
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