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Master Cylinder - WTF?
Not to diss the 'pressure' bleed method, but I'd like to add a rather important point.
On the submarine, some of my gear included some very sensitive detection equipment intended to measure flow rates. (sorry, I have
to be vague about this stuff on the list). The system was over-engineered to withstand depth charges, missile launches, you name
it. It worked, in many ways, like our braking systems, and had high-point bleeder valves throughout, including a design that makes
it continuously vented. It was connected directly to a high pressure system (relation: guy in car pumping brake pedal). You would
think that to bleed the air out of this system, you just pop open the bleeder valves and take advantage of the pressure that was
already there. Nope. Instead we would go through hours of work to isolate the system, just to add another water source (at ambient
pressure (relation: brake reservoir)). At the vent lines (relation: bleeder valve), we would connect a vacuum pump. We would then
draw the source water into the system (at a vacuum) to vent it.
So, why go through all of this trouble when we could have just popped the vent at pressure? Small bubbles don't like leaving, and
just love staying put. Bubbles that would be too small to leave the system at atmospheric pressure will expand under even slight
vacuum, resulting in _significantly_ greater removal of air from the system. You would think just venting at atmospheric would be
good enough, since the bubbles would surely collapse under the several hundred pounds of operating pressure. Not so.
So, do yourself a favor and pick one of these up at your local flaps:
http://www.jeephorizons.com/tech/brake_bleed.html
This turns brake bleeding into a one person job. You connect it to the bleeder, draw fluid through until air free, close the
bleeder, and move onto the next wheel. The only concern is drawing the reservoir dry, and to compensate for that, put a tranny
fluid nozzle on the brake fluid bottle and invert it into the reservoir - it will auto-maintain reservoir level at the bottom of the
nozzle opening.
Side note: Even though full stroking a new MC can possible damage the seals, it does have to be fully stroked at least a few times
to properly lubricate the seals and expel trapped air. The safest way to accomplish this is by bench bleeding.
HTH (everyone)
Al
(still fighting for the ability to vacuum fill on the big boy toys)
> -----Original Message-----
> From: Dan Bubb [mailto:jdbubb@verizon.net]
> Sent: Thursday, March 29, 2007 3:09 PM
> To: Daun Yeagley; scirocco-l@scirocco.org
> Subject: Re: Master Cylinder - WTF?
>
> Well, just to throw in my >.02.
> I went through the same thing when installing a new MC on my
> Scirocco. In the end the MC was replaced under warranty and I
> bought a power bleeder and never had any problem after that.
> The assumption was that even a new MC can be damaged stroking
> it full stroke like you would if you were bleeding the old
> fashioned way.
> In your situation the first thing I'd do is run over to Ron's
> and borrow his power bleeder. ;^) If that doesn't do it then
> I'd see if the MC can be replaced under warranty.
> Needless to say the power bleeder is a life saver if you have
> A3 rear calipers that have to be mounted with the bleeder
> screw on the bottom. It's still a damn good investment for
> just about any other bleeding situation.
> Dan
>
> From: "Daun Yeagley" <vwdaun@yahoo.com>
> To: <scirocco-l@scirocco.org>
> Sent: Wednesday, March 28, 2007 11:31 PM
> Subject: Master Cylinder - WTF?
>
>
> > As some of you may know, I just installed a new master cylinder on
> > Deiter, the '81 S. It seems as though the old one was
> leaking fluid
> > into the brake booster. And yes, I replaced the brake
> booster with an
> > unknown quantity used booster. (Can't seem to get a new one now.)
> >
> > First attempt at bleeding the brakes resulted in the semi-expected
> > snapped bleeder screws in the rear, so I replaced both
> wheel cylinders
> > this evening.
> >
> > Filled the brake fluid reservoir and started at the right
> rear wheel
> > to work the air out of the system. I don't have a pressure
> bleeder,
> > so I'm stuck with the old "someone pump the pedal while I turn the
> > bleeder screw" routine. After working my way around the
> car, I still
> > have a brake pedal that shows no real resistance to going to the
> > floor, AND it seems to have a "catch" in it about 2/3 of
> the way down.
> > (Which was present when I started.)
> >
> > So now what? Have I done something wrong here?
> >
> > Daun Yeagley - Wilmington Ohio
> > '81 Scirocco S (Deiter) '86 Scirocco 8v
> > '88 Scirocco Slegato (Gino!) '88 Scirocco 16v
> > '96 Passat GLX Wagon '56 Cessna 172
> > The first rule of intelligent tinkering is to save all the
> parts - Ehrlich's Law.
> >
> >
> >
> >
> >
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