Pump Tubing Questions

LittleStace

Hi there!  I'm new to this board.

I have a Medela PIS, and I've had milk back up into the tubing at least a few times recently.  Any idea what causes this or what I'm doing wrong?  Also, once in a while when pumping I hear one side or another sound like it's leaking air.  I don't think there's a hole in the tubing since it doesn't happen all the time.  Am I just not tightening the parts well enough?

Thanks for your help!