I've also been chasing leaks on the low pressure side of the brake system where the low pressure tubes go into the top of the master cylinders. This tube is a thin walled tube and the plans have you install a small hard plastic tube into the end to give the ferrel something to bit down on. It didn't work very well for me.
I found this part at Lowes in the plumbing area.
Untitled by
akarmy, on Flickr
It has a much longer brass sleeve to install in the tube and seems to secure the connection better. Time will tell if it stops the leaks.