OK guys, sorry but I cringe at some of this thread and would hate to see some one go through contaminated fluid in the system
Brake fluid eats all sealants over time, this is why sealant is never used on any brake system (auto shop 101).
Proper assembly is essential to prevent leaks.
The VW bung is exact match to factory part, and should be lubed with brake fluid or brake assembly lube then inserted in clean bore, pipe angle should not stress or pull the rubber(slotted washer is optional). It can take a little finessing to get the pipe to sit neutral.
Millions of VW's used this seal this way with no issues, but bad quality rubber can also cause issues.
FWIW, I have found the best VW cross over master cylinder is the early(till 57) type as they are 19mm piston and same over all length, the later ones are 17mm piston (and slightly longer) and result in higher brake pedal effort(you will also have to drill out mounting threads and plug 2 ports on either VW master). Plus side is they are inexpensive and readily available in a pinch
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