Okay! My experience with Cryogenics is pretty limited but "Diamond film"? Did not know that would increase thermal continuity.
Have you considered Silver Solder or is that what you meant by Brazing?
I have used Silver brazed Copper componets to contain liquid Nitrogen (with out pressure) on occasion with out any problems but not for cyclic operations and no great length of time (over 5 days) and we never got lower than -200 F.
None of the joints failed though.