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During the priming process, the pump needs to push the air out of the pump basket into the return line. If there is a suction leak, it will continuously draw air into the pump basket and out the returns. So the continuous air out of the returns is a good indication of an air leak.
You are correct though in that it the pump can operate properly with a small amount of air. But if there gets to be too much air, it can cause pumping issues. A short run at higher speed should take care of that.
sj,
It seems that while you may have some air leakage, you may also have some restriction that is greatly reducing flow making it difficult to clear the air from your pump basket. I would definitley clean the filter as soon as possible, I'd also open the pump basket to look for any blockage in the impeller (after disconnecting the power of course). I'd then clean the O-ring and its mating surfaces really well and inspect for any damage (scratches etc.) before putting it back together.