You could try to put some treats on the feeder, getting them used to it is all about desensitizing. I would go inside and open and close the feeder by pressing the lid until you get bored and want to leave. That's what I did, now I can drop sheet metal onto sheet metal next to my chickens when they are sleeping and they dont even move, the reasons is because they are desensitized to loud sounds because of the loud treadle feeder.
Today they are perfectly fine with it and trained to it.
You can also put a heavy rock on the feeder for about 30 mins a day to let them eat only during that 30 mins, its a little cruel and they will be volatile while hungry but it keeps the sparrows away, it can help you with the sparrows and keeping it open while training them
msg me back for any updates!