Date: Wed, 23 Feb 2005 13:38:47 -0500
Reply-To: Jonathan Farrugia <jfarrugi@UMICH.EDU>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Jonathan Farrugia <jfarrugi@UMICH.EDU>
Subject: Re: Theory: Clutch Master Cylinder
In-Reply-To: <d78baa92a419a4615c1cb8557affc76b@knology.net>
Content-Type: TEXT/PLAIN; charset=US-ASCII
see below for ***Replies***
On Wed, 23 Feb 2005, Jim Felder wrote:
> 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.
***Reply***
even a grossly failed clutch master cylinder will not effect the brakes
unless the brake reservior gets extremely low. for that to happen you
would normally need a brake leak also because the clutch master cylinder
supply take off is above the brake master cylinder take off. it seems
specifically engineered this way to protect against brake failure in the
event of massive clutch system failure. although i think i have a decent
understanding of brake system operation and theory, my theory didn't come
from theorizing. i've taken apart several failed clutch master cylinders
and documented what i saw in all of them and that was the aluminum
cylinder riding against the cast iron bore thus making metal particals.
most people are going to drive like i have, past the initial failure
point. because 90 percent of failure (i.e. leaking) is going to start
while you are on the road and you are going to drive it home not call a
tow truck. most people won't even notice the leak until it drips on
their shoe which means that its most likely past the initial failure point.
> > 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.
***Reply***
i agree there is neglagable or no real fluid transport through the system
with a properly functioning hydraulic brake/clutch system. metal
particals are generated, i measured them :). these are the ways that i
know that fluid can move through the system thus transporting metal
particals from the master to the slave.
1. if you were to have a leak at the slave.
2. if you drain the system like i outlined in my proceedure by clamping
the supply line and opening the slave bleeder.
3. after install of the new master residual metal in the part of the line
closest to the master will be pumped to the slave. this is certainly the
most tenuous of the ways but i still believe it exists although i find it
difficult to quantify.
> > 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
> >
>
>
>
