I connect my leaders with a Double Uni. I make sure that the larger diameter line is wrapped atleast 6 times around the thinner diameter line and have never had an issue.

1) The reason for the 6 times rule is because of something I read a while back from one of the line manufacturers staing that with the smaller diamter braid being attached to the larger diameter flouro, that the flouro would need to be wrapped 6 times around the thinner diameter braid to assure that the knot does not slip.

2) Also, when using flouro, make sure that you wet the line before cinching the knot down, this is critical. The friction created from the lines tightening down on themselves is enough to cause heat and abrasion damge to the knot making it much weaker than the advertised rating. I know some fishermen have actually used vaseline on their knots to keep the friction low when tightening down the knot.

3) Also, make sure that you are getting Flouro LEADER material for use as leaders. They are designed with different charateristics than regular flouro. So many anglers got on the "Hate Berkley Vanish" bandwagon because it would snap easier than some other flouro's when used as a leader. Berkley actually had Vanish Leader Material that was constructed differently than the regular Vanish Flouro, but nobody wanted to pay the premium price for it adn instead used the regular line and had poor results. The actual Vanish Leader Material is more expensive, but it is AWESOME line. Look to see if you can find specific flouro leader material if you are purely using it as a leader, there is a difference.