Sometime, after some years, the glue used by the original manufacturer tends to dry leading to the partial detachment of one or more leading edges valves. Here's how I fixed my leading edge bladder regluing ALL of them. (for my Slingshot 2006 TurboDiesel kite)

Step 1: Detaching

After removing the bladder from the kite body, warm up the valve with the help of an hair drier.
The original adesive (or glue) eventually will soften and the valve will detach.

Step 2:

With the aid of scotch tape, make the bladder to stay flat on a plain surface.
Notice the "bumping" of the urethane where the original valve was glued.
If you pull too hard during the detachment, the bump will be more evident, so be gentle and allow the heat to do the job.
We will take care of the bump later.

Step 3:

It's time now to take care of the deformed end where attach the valve to. We need some round shim to slip under the bladder. Looking around in my place I came up with a sanding disk for my angle grinder which had the right size and "thickness"

Step 4:

After checking eveything looks ok, let's add some weight to the valve and glue it.
In my case I've used some large bolts inserts as weight and Aquasure as a glue. It holds pretty well
The pen inside its only to prevent the stack from falling over the bladder.
I've als used paper tape to keep the inserts together.
If one insert falls on the bladder it would pierce it, so, be careful.
Be patient. Aquasure takes 12 hours to cure.