Date:         Wed, 23 Feb 2005 09:13:23 -0600
Reply-To:     Jim Felder <felder@KNOLOGY.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Jim Felder <felder@KNOLOGY.NET>
Subject:      Theory: Clutch Master Cylinder
In-Reply-To:  <Pine.SOL.4.58.0502230054110.13096@zektor.gpcc.itd.umich.edu>
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>

Jonathan,

I have to question this theory a little bit, at least for those who
might end up replacing a slave cylinder when what they really need is a
proper bleeding. And a second bleeding in a few hundred miles.
>
> Now for some theory.  You will often see that people end up replacing
> the
> slave shortly after they do the master.  One possible cause for this is
> not cleaning the slave or flushing it adequately.  As the master failed
> the failing seals allowed the aluminum piston to scrap against the
> bore of
> the master cylinder.

They seals don't keep the piston from scraping the cylinder walls, the
lubricating qualities of the brake fluid do that. At least in the first
stages of failure, and that's what we're probably talking about here.
Anyone with a system degraded enough to have metal shavings coming out
of the master cylinder probably isn't driving the car and knows not to
if not by the lack of brakes, then my the fluid all over their shoes.

> This generated metal particles in the system, which
> accelerated wear on other parts of the system.  So you need to get all
> that metal out or it will prematurely wear out the parts with the
> weakest
> link being your slave.

Even if these particles were generated, how would they get back to the
clutch, what would transport them? It isn't a circulatory system like
power steering or oil. It's a static pressure system, and the fluid
moves back and forth only the amount needed to actuate the clutch
lever. If fluid is leaking out the slave cylinder and lowering the
level in the brake reservoir, then yes, it's moving metal particles (if
there are any) toward the slave cylinder. But the chances of that
happening are very small. It takes real effort to get air bubbles from
the front to the rear, lot of work. I don't think it's likely to happen
on its own.
>
> Then I rebleed the system in a
> couple of weeks once all the parts have been moving around for a while.
> This is done to help remove any metal that you might not have been
> able to
> get out with the initial clean out bleeding.

This is the key. I recently posted a detailed account about how I had
read this and almost didn't heed it. Conventional wisdom says that
right after your master cylinder fails on a vanagon, your slave
cylinder will go. I maintain that what is actually happening is that
you replace the main, and don't do the second bleeding on the slave in
a few hundred miles. You know the master is good, you figure the slave
is bad. You replace and bleed. It works. You think you had a bad slave
because the bleeding is what was needed.

Jim
>
> jonathan
>