err... a few years ago I got lazy (I've cut down and re-attached a lot of lines over the years) and started to attach my lines at the bar ends without re-sleeving them at all (the old Q-power lines spoiled me I guess). Just fold over the last 12cm of line or so and then make a loop using the figure 8 knot. When you cut down your lines, you need to keep the sleeving at the kite ends so you can attach and detach to the pigtails easily. But at the bar end, whether I've attached the unsleeved ends to a knotted leader line or to a metal ring, I've yet to see a line break there. Seen them break plenty in other places though. Sflinux says that he has seen lines usually break at the stock sleeve, and so have I. Sometimes I wonder if the holes needed to sew a sleeve onto the lines weaken them at that point.
Go ahead and cut your other lines shorter to match the broken one and enjoy kiting on shorter lines... see my post at viewtopic.php?f=1&t=2365823
To do this, put a screwdriver through the loops of your lines at the kite end, stab it in the ground, pull all the lines tight, then bend them all over in the same place. Hold on to where the bend is and then make your loops there, final check your lengths before cutting off the excess. That way all the lines are the same length. It doesn't really matter if your loops are of a little different size because the length from old loop end to the one of the new loop you've just made will be the same on all lines.
Oh, and be careful about using and re-using your lines this way--- eventually as the lines get older they will get weaker. When I break a line on my own kite under normal wear, I replace all the lines, as I've learned that another break on the same set of lines is soon to follow!