Thanks for the information! As far as I can tell, I am following the procedure in the Baxter manual exactly. But when priming occurs, the system is in the state you mention, i.e. the transfer set is not yet connected and the clamp on the patient line is open. Also, the cap at the end of the patient line in the organizer is not airtight, so I wonder if air is being drawn in during priming? I also noticed that the sides of the drain bag can stick together, creating some resistance to filling. Could this produce the negative pressure you describe, or allow bubbles in the drain line to back up into the patient line? Thanks again for your feedback.