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Re: [T3] Master cylinder question

On 11 Nov 2001, at 16:38, Everett Barnes wrote:

> Hi, I need to replace the master cylinder in my '65 Type 34.  I picked up a
> couple NOS FAG master cylinders recently:

> 1. 311 611 021A - Brake master cylinder - 20.64 - single circuit drum
> brakes - to chassis # 221 974 and 365000001 - 365220883 for models 361-368
> (Squareback)

> 2. 311 611 023 - Brake master cylinder - 19.05 - single circuit disc
> brakes - '66 or '66-'67 only, depending on the model  (Type 34s went to dual
> circuit in '67, a year earlier than the other Type 3s, unless they had
> option code M139)

I had always thought that ALL VWs got the tandem MC in 67, but I 
found M139, just as you described it. Interesting.

> My parts books lists my original master cylinder as 311 611 021B - single
> circuit drum brake - 19.05

> Question:
> 1. Can I use either of these master cylinders?  I have been told there is no
> problem upgrading to the Squareback 20.64 master cylinder by one shop I
> talked to.  They said they actually only sell people the 20.64 one.
> 2. What's the difference between the 19.05 drum and disc brake master
> cylinder?  There appears to be no physical differences.  Internal valves?
> 3. Should I disassemble and lubricate the NOS master cylinder with brake
> fluid before installation?  Both are in perfect external condition and the
> pistons moves smoothly when bench-tested..

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.   

You could look at the two MCs and see if the one made for front 
drums has some kind of extra valves screwed into the two front 
drum ports. If so, you could transfer them to the 19mm cylinder.   

OTOH, I have never been able to understand what those valves 
really do, and I note that when VW came out with the tandem 
cylinder they didn't bother to use the residual pressure valves which 
had previously been considered "necessary" for a drum brake.   

I have pondered this question for nearly 30 years and still don't see  
that they serve ANY useful purpose. If it were my car, I would 
install the 19mm MC without any extra valves.   

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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