The light I have (LED) seems to work very well with the dye I bought, based on a quick test I did. So you mention a valve. There at least 3 that I am aware of, the front and rear expansion valves, 1 per evaporator, and the pressure relief valve on the compressor. The compressor checks out. So I assume it one of the expansion valves, buried somewhere under the dash or behind the interior panels in the back of the van. Fun!
Thanks for the tip on where to look.