[T3] Master Brake Cylinder

Jim Adney jadney at vwtype3.org
Wed Nov 15 19:35:51 PST 2017

VW used at least 5 different designs of tandem MCs from '67 to '73, 
from 2 different makers. As time passed, VW only supplied the latest 
version for replacement into earlier cars, so there were often 
accomodations that were needed.

Beetle and Bus MCs are physically different from ours. For example, a 
Beetle MC has tapped bolt holes in its mounting flange.

The correct MC for your '71 has the 2 brake light switches on the 
driver's side and an angled port on the front to feed the LF caliper 
via an odd brake line that loops around under the MC and out to the 
left. This is the replacement configuration for all Type 3s with 
tandem MCs, unless the original can be rebuilt.

The MC you have with one switch in front was for '70 only. If you 
compare the size of the drilling at the bottom of that port to any of 
the ones on the right side, you will see that the switch ports have 
larger drillings at the bottom. They work best with switches, since 
they allow air to get out of the switches better when bleeding.

Old MCs saved and stored wet with brake fluid are seldom good 
candidates for reuse without honing. Take one apart and see what it 
looks like inside. Look at the condition of the rubber seals. I 
suspect you'll discover several of the reasons for rebuilding. 
There's some chance that you won't even be able to get both pistons 
out, because of rust in the bore.


On 16 Nov 2017 at 1:45, J. Jonik wrote:

> Re '71 Sqbk.  Master brake cylinder is obviously shot.  It's a wonder it stops the car, but it does.   Unexpected benefit is that one DOES drive Quite Carefully with minimal brakes.
>   I have four or five spare master cylinders, all removed from past cars when they were working.   BUT...there's differences...besides some being more rusty than others.
> Just one has an electrical plug connection at the end (facing front) of the cylinder, not on the side.  It looks like the electrical plug has enough room so it won't bump into the axle beam.  Was that from a bus, or beetle?
> Some others have electrical plugs removed.  Others have plugs, but in different plug holes than the others OR the one still in the car.
> Bottom line question:  Does it matter which hole takes an electric plug...or a brake fluid line?    Interchangeable? 
>  That is, I'd rather have the three brake lines going to the same hole arrangement as the one in the car, to minimize metal tube bending contortions....then just put the electrical plugs in the remaining two holes.   Is that the right path?   Bentley says nothing about this.

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

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