Re: [T3] Ill-fated master cylinder?????

["Jim Adney" <jadney@vwtype3.org>]

On 13 Sep 2000, at 9:54, Adriaan Loedolff wrote:

> Jim Adney wrote:
> 
> > (Snip).........sort of like the ill-fated 68-9 master cylinder.
> 
> Please explain further - I've not heard of this before.

In the beginning the master cyl was a single circuit with a single 
SPST (single pole, single throw) pressure activated brake light 
switch.

In 67, US law required tandem MCs and VW introduced a normal 
tandem MC with 2 SPST pressure activated brake light switches. I 
THINK that most parts of the world got this tandem cylinder at this 
time also.

In 68, US law required VW to add a feature to the braking system 
that would indicate to the driver that there had been a brake failure 
of some kind. Their first attempt at a design was used for 2 years, 
68-9. This master cyl had a second 1/2" cylinder under and parallel 
to the first with a shuttle piston in it.

Under normal conditions the shuttle piston received pressure 
equally from both ends, applied from the front and rear braking 
circuits. But if one circuit failed there would be no pressure on one 
end and the shuttle piston would move, actuating a switch screwed 
in the side, which would turn on a light on the dash.

So this cylinder had 2 SPST pressure activated brake light 
switches and 1 mechanical switch for the brake failure light.

This MC was expensive and complicated and hard to rebuild. With 
the shuttle piston on the bottom, it also had a tendency for all the 
water and crap to sink down there and clog things up. It had more 
parts than any other MC I have ever seen.

In 70 they came out with a new MC that was exactly like the 67, 
except that they just used 2 SPDT (single pole, double throw) 
switches. Clever wiring of these new switches performed the same 
function as all the earlier 68-9 contrivances.

68-9 style MCs have probably not been available for over 25 years, 
and that is a GOOD thing.

If you have one of the 68-9 cars you can upgrade to the later 
system with a later MC with the DPDT switches and a MC wiring 
harness that runs from the MC to just above the LR corner of the 
gas tank. You may also need to replace the LF steel brake line, 
since the latest MCs moved the outlet for this pipe (as of 71.)


-
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054

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