1) Find the leak. Pump up your kite and listen for any hissing sounds. If you don't hear any, put the kite down under water (swimming pool, calm body of water, etc) and look for bubbles rising to the surface.
NOTE: Check the valves first, Unless you have been crashing your kite into trees, rocks, or a shelly beach, you shouldn't have random holes in your kite. The glue around valves tends to go first.
2) Remove any obstructing hoses from the valves, then find nearest opening to remove bladder.
3) Pull out bladder past the point where the leak is occurring.
4) Remove the valve by pulling gently, trying not to stretch the kite bladder in the process. If it doesn't come off easily, place the valve in a bowl of hot water to loosen the glue, then remove it.
5) After cleaning the surface of the bladder and valve with alcohol wipes, apply Aquseal (from a local dive shop) to bladder, being careful NOT to get adhesive in the hole itself. (NOTE: if you want to avoid having to use glue, try a self-stick valve -
KiteFix.com )
6) Place valve in place, being careful NOT to smear glue into the air hole.
7) Place a circular object over the valve (my favorite is a small piece of PVC) and something to weight it down (brick, piece of wood, etc.)
8) Leave the kite in a safe place for 8-12 hours, then reassemble. Pump up kite and leave inflated for 30 minutes to make sure the bladder is not slowly deflating (which is a sign that you didn't correct the problem, OR that you may have ANOTHER leak. ... in that case, go back to step 1 :-)
Kiss The Sky Kiteboarding.com