I have recently gone through several master cylinders on my '92 Eurovan.
The original failed in one of the cylinders, giving a long brake pedal. I got a rebuilt unit from Canadian Tire and that is where my fun began.
The first replacement failed in a couple of days by leaking from the master cylinder into the booster. I took it back and was told it was defective and got a replacement. One week later -same thing with the replacement.
Figuring that this was not a coincidence, I took it in for a mechanic to look at it. He couldn't figure out anything so he called a VW specilialist buddy of his. The story he got was that the rebuilt units almost always fail in the Eurovan - the seal they use is not up to the pressure. The solution get a new one from Volkswagen or good used one from a wrecker.
So be forewarned and save yourself some aggravation if you face the same job.
'92 Eurovan '90 Jetta '90 Fox '88 Fox '87 Cabriolet '78 Rabbit '72 Bus '72 Thing