milk in pump tubing...what am I doing wrong?

3litttlebirds

I have used 2 different kinds of pumps, so I know it's nothing with the pump specifically....but I frequently get milk drawn into the tubing. Does this happen to everyone or am I doing something wrong? The membranes are fairly new. Tubing is new. Milk isn't reaching the top of the bottle and overflowing or anything. Any ideas on what to do so this doesn't happen one third of the time that I pump? Thanks.