I've upgraded the standard master cylinder to a dual master cylinder because I am swapping out the skinny little early-'57 brakes for CB Performances Wide-5 Disc Brake kit.The original plan was to use a Volvo reservoir on-top of the new dual master cylinder (lots of people on The Samba are doing it so it must work, right?).  Needless to say the outlets on the Volvo unit are too big for the FTE master cylinder I purchased, I've since learned that different brands have different size inlets. 

So off to Pick n Pull I go.  I had also heard that a Rabbit reservoir would work so that's what I sent off to find.  The VW unit plugs right in with no mods!  While I was snagging the reservoir, being sure to take the seals as well, I also grabbed a spare Brake Switch as there are two required on the new master (it won't be used, but it plugs the hole and if the original fails I'll have a backup).  I found the VW reservoir on Rabbits, an early Scirroco, and some Foxes and Jettas that were in the yard.  Most had a cap with a sensor built in, one of the older cars had a plain cap,  and the cap from the Volvo reservoir works as well (at least it's good for something).  The one that I'm using is part #171 611 301B as one of the other reservoirs I picked up at the same time was a little too close to the forward brake line (see below).

Here's the VW reservoir (left) compared to the Volvo (right).