Hi Dave,
I don't pay any attention to my modern cars, but it sure looks to me as if the
brake and clutch MC
on my TR6 are vented as is the brake MC on my MGB. It's not so clear about the
MGB clutch MC.
But even if the system is not vented, there is still air in the MC, and I think
everyone is saying
that there is sufficient moisture in the system from whatever source(s) to
saturate the DOT 3/4 thus
leaving droplets of water in the system and thus rusting the brake system
components.
So I think my question stands. If we are driving around with the DOT 3/4 in our
brake systems
saturated with moisture, why be concerned about a can of brake fluid that was
opened for a few
seconds and then resealed?
Sells more brake fluid if they convince us to throw out the reminder of the
opened can? :-)
Don Malling
Heglund, Davis (MN17) wrote:
> 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.
> ***************************
>
> 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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