When all is setup correctly I have not see air enter the system. About the only place it could be generated is if the pump got hot enough to boil the water in it. This would cure it's self when cooled down.
The 1s only has to push past boost pressue. On the 2c,2d and 2s we maintain 7bar in the line. So lets say the pump is running the pickup get uncovered and air gets into the system. The air will compress and not pass out of the pump. The pump will continue to run but it never clears it self because of the 5bar pressure line pressure. If the pump had a much greater volume for each stroke it might.
On most occasions the boost incountered by a 1s is only about 1 bar.
If the pump is accidentlly inserting air the pump would have to be defective.
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Brad
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