After some months of experimenting (and a lot of tossed away evaps), I managed to braze two evaps in an oven. They came out very nice and needed no further cleaning. I always hated having to clean the much oxidized copper only to hope that there will be no leaks. Also I did not like the idea of leaving corrosive flux behind inside the evap when brazing with silver rods.

Now I dont need any flux at all...

Regards,

Philipp