I have been having letdown issue when I try to pump. I have an older used pump that I got from a friend. When DD nurses, I dont have an issue with letdown, so I have been pumping one side when she nurses in the morning. The only time I cant get a letdown is when I try to pump, so I am wondering if something could be wrong with the pump. I changed the membranes but that didnt work. I don't want to go out and buy a brand new pump and continue to have these issues. I'm so confused!