> Alan, Ben and list
>
> I have also had this problem and it always seems to be due to
> a very slow leak like you mention.
>
> Also....
>
> The main brake specialist in Adelaide tells me that even on
> sealed systems such as you mention there will still be water
> uptake in the fluid. He said there has been much debate on the
> problem over the years and it has always been blamed on the
> breather hole in the master cylinder cap on the older systems.
> But when the industry went to sealed systems thinking that
> they had licked the problem they found much to their surprise
> that it was still happening. He states that it is now
> becoming an accepted theory that MOST of the water uptake is
> actually through the flexible rubber brake lines from
> atmosphere. I suppose this must be something along the lines
> of osmosis?
I think that's very interesting and worth checking out. There is no doubt that
most rubber compounds are not totally impervious to the passage of smaller
molecules. Perhaps steel-jacketed teflon hoses would fare better. I also wonder
if there's any available moisture in the rubber compounds themselves.
As you know, in some cases, moisture finds its way into the brake system as a
result of pouring in brake fluid that has been sitting around for some time in
an open or partially filled can, absorbing water.
Here's a strange theory that I just concocted that might also explain part of
it. See what you think:
The dust caps on the wheel cylinders rarely seal very tightly against the nib
of the brake shoes (or the brake push rods, depending on the system). When the
brakes get wet, some water or water vapor could make its way into the space
between the dust seal and the piston, coating a bit of the walls of the
cylinder. When the piston moves in and out, small amounts of water could work
its way into the wheel cylinder and then be absorbed by the brake fluid. If
there is any slight scoring or pitting, it would happen even faster. Multiply
this by the thousands of times that you splash through puddles and apply the
brakes, and it might just begin to add up.
Is that stretching it a bit? Or does it sound reasonable? I do know that the
pointy and rather rough rectangular ends of the brake shoes on my M37 don't
create a hermetic seal with the dust cap, so who knows?
Regards,
Alan
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