The meter insisted on leaking...not much, but enough to put water in the pipe. I make a swab of pieces of a towel attached to a heavy wire...coat hanger I believe..
I had to replace a garden faucet once, the pipe was the uttermost low point of the plumbing in the house. Closed and opened everything, waited six hours, no matter what there was a slow drip. I used a piece of bread.. mashed it up into a semi-solid bit of dough, stuck it way up the pipe. Once the water is back on the bread flushes out quite easily..
interesting thread. 'In God we trust, everyone else needs to show their data'.
From a theoretical standpoint I'd be inclined to agree with hackney - if taken patiently and the joint heated sufficiently before starting the solder, the pressure should have had time to equalize between the pipe and surrounding air. His experiments seem to confirm this..