[T3] Master Cylinder workover

[Keith Park]

i just pulled apart the brake system in my Tbird, I re-did that in 1987 and
converted to DOT5, I did NOT rebuild the master then, and I dont think its
ever been done since '57, but at least as far back as the early 70's.
Lots of dirt and rust in the master, from the old DOT3, worn seals but no
leaks.  The rest of the system that was new in 87 with the DOT5 was still
like new, and this car has seen very little use.

my 71 still has the master I put in it in 1988 when I got it, still the DOT3
or 4 as I didnt want to keep eating the brake light switches and havent
converted yet.  Still seals fine, but Ive had to rebuild calipers and wheel
cylinders over the years with the non-silicone

Keith


Topnotch Restorations
topnotch at nycap.rr.com
http://www.a383ina68.addr.com/radiorest/main.htm
71 Squareback  "Hothe"
65 Notchback  "El Baja Rojo"
93 RX7  "Redstur"
87 Golf  "Winterat"
 
-----Original Message-----
From: type3-vwtype3.org [mailto:type3-vwtype3.org-bounces at lists.vwtype3.org]
On Behalf Of Jim Adney
Sent: Wednesday, August 19, 2015 4:12 PM
To: type3 at vwtype3.org
Subject: [T3] Master Cylinder workover

I've had one of my '73 Squarebacks since 1990. When I bought it, I 
rebuilt the calipers and installed DoT-5 Silicone Brake Fluid in it. 
All it's brake parts, except for the front pads, are still the 
originals.

Sometime between 2000 and 2005 I noticed that the brake fluid level 
seemed to be slowly going down, and I found that the master cylinder 
was leaking out the front end, onto the driver's floor. This is 
pretty typical for old MCs, so I pulled the MC, intending to rebuild 
it. What I found was that this was a FAG MC, the seals were still 
fine, but there was a little rust in the bore, just inside the mouth, 
where the seal needed to seal. The rust was pushing the rubber out of 
the way, allowing brake fluid to leak out.

I decided to take a chance and just clean up the front end of the 
bore, so the seal would have a chance to do its thing. I never 
actually took the rear piston and seals out. I just did a minor 
amount of work and put it back together with all its original 1973 
parts.

This spring, once winter was over, I found that the brake fluid 
reservoir was empty, at least one side was. I refilled it and drove 
it a bit, but it was scary since there was air in the system, meaning 
that I had to pump the brakes at least once to get decent braking.
So today I pulled the MC out to see what had finally happened after 
all these years with the original parts.

What I found was pretty amazing. There was very little wrong inside. 
I had expected to find more rust inside the mouth, but the bore was 
perfect. The seals all looked almost perfect, except that the very 
first one had just enough wear on its rim to explain a small leak in 
very cold weather.

What happens is that in cold weather everything shrinks, but rubber 
shrinks more than iron, so the seal shrunk and pulled away from the 
bore of the mc and let the fluid out. This probably took weeks, and 
those of you who never see really cold weather will probably never 
experience this. It got down to -10 F here last winter. That's 
certainly not record cold for us, but it's cold enough to cause a 
problem with a worn seal.

Now keep in mind that this seal was used with standard fluid for 17 
years, and standard fluid is not nearly as good a lubricant as DoT-5, 
so most of this wear probably occurred prior to 1990, or by 
2000/2005, when there was rust in the bore there. So you can see how 
good those OG parts were. And you can also see why I am a strong 
proponent of DoT-5 Silicone Brake Fluid.

You can buy DoT-5 at your FLAPS. If you have questions about it or 
about how to install it, email me directly for my writeup on it.

-- 
*******************************
Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
*******************************

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