Having been bleeding one of my front calipers today, I came to the conclusion that air can get in round the base of the bleed nipple if it isn't pinched up before you release the lever. I was using a valved bleed pipe and just pumping away
, and noticed the continued presense of a bubble or two. Once I did it the hard way; couple of squeezes to pressurise, then hold, open the nipple, close the nipple, release, the bubbles stopped.
Why was I bleeding one caliper? It was seizing on. I found one of the pistons wasn't moving properly. I ended up pumping it out and, cleaned the aluminium salts off and reassembled. Ah, much better now.