Joy,

I have been renting lactinas since they first came out.  I have never had one
break or not work.  I have, however, seen the piston hesitate when the gasket
is new.  Some  things to try:

1.  Is the gasket on correctly?  Is it lying flat against the piston?

2.  Let the pump run for awhile to 'break in' the gasket.  If the pump still
hesitates, try the gasket in the replacement parts bag of her kit.  Since you
tried a different piston already, how about trying a different cylinder?

3.  Try soaking the gasket in hot water, then, while it's still hot, put it
on the piston and let the pump run.  I've had this work with stubborn
gaskets.  I learned this from a Tupperware dealer who uses this technique
with hard-to-put-on lids.  This is not Medela-approved, so ask them first.

I don't think this is a lactina issue.  I think it's the kit.

Elizabeth Puzar, IBCLC   San Jose, California