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Re: [T3] Master cylinder question

<x-charset iso-8859-1>Of course you are correct in this matter, Jim, I read further into my early
parts book and found ALL the wheel cylinders for the Squareback are larger.
The rears are much larger -  20.64mm vs 25.4mm  (sizes vary slightly by

I can imagine some road rally racer in '64 outfitting his new "S" model
Notchback to the "huge" Squareback brakes... LOL.  Or just wait a few years
and get a disc brake front end.


----- Original Message -----
From: "Jim Adney" <jadney@vwtype3.org>
To: <type3@vwtype3.org>
Sent: Monday, November 12, 2001 7:51 AM
Subject: Re: [T3] Master cylinder question

On 11 Nov 2001, at 19:46, Everett Barnes wrote:

> Everett said:
> >> 19mm vs. 20mm master cylinder

> Jim said:

> >If you switch to the larger MC you will reduce the stroke necessary
> >to apply the brakes, and make the brakes feel "firmer" but you will
> >have to press harder to get the same amount of braking action,
> >because you have actually reduced the hydraulic mechanical
> >advantage by 15%. This strikes me as a lot, so I wouldn't do it.

> Since the 20mm was stock on the Squarebacks, wouldn't this mean the
> Squarebacks had  brakes with the attributes you mentioned?  This seems
> disadvantageous, I had assumed the 20mm m/c gave you improved braking
> somehow to allow for the hauling capacity of a Squareback.  I wonder why
> would do that...

I'm not that familiar with the early braking systems, but I suspect that
VW changed the MC diameter they also changed the wheel cyl dias. You
could certainly change both dias to keep the hydraulic advantage
approximately constant, but sticking with the 19mm MC is the easiest, and
best, way to go.

The thinking that increased dia makes for better braking is widespread in
automotive public. For years, aftermarket suppliers for the Porsch 914 have
sold the 914/6 19mm MC as a high performance upgrade for the 914/4 which
came with a 17mm MC. This is completely backwards and results in exactly
the opposite effect.


I think some of the early squares came with 15/16" (23.8mm) rear wheel
cyls. It was probably these cars that came with the 13/16" (20.64mm") MC.

On all of these, you will often, depending on the manufacturer, find the
diameter cast into the outside of the part to the nearest mm, even though a
careful measurement will make it clear that the bore is an exact number of
sisteenth's of an inch, and not any integral number of millimeters. If you
the ATe literature that comes with your MC kits you will find the bore
out as 19.05 which seems like a strange number until you realize that this
exactly 3/4". ATe casts a 19 on the side of this MC.

Jim Adney
Madison, WI 53711-3054

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