Don Foreman wrote:

Lesson learned: clamp lightly before brazing. (I just used gravity)
The air in the hole must have expanded and pushed it out a bit before
the brazing alloy froze.

I had a scary experience, years ago, silver brazing a snug fitting stud
into a blind hole drilled into a piece of stainless. I had put water
soluble flux into the hole, pressed the stud into it and was heating it
up to make the braze flow. The stud flew out of the stainless work held
in a vice with a bang, past my head and embedded itself in a ceiling
beam. Now I always drill a breathing/venting hole when possible.

cheers
T.Alan