Rear Aux Unit - Bad expansion valve?

I have a 2001 Windstar 3.8L with a rear auxiliary heat/ac unit. I replaced a few components on the ac system including the expansion valve on the rear aux unit. Shortly after installation and recharging the system we heard a faint hissing sound from the rear unit. We don't use the van much now that the kids are grown and rarely does anyone sit in the back. My son drives it mostly and I asked him to keep an ear out for the hissing sound. He apparently ignored the noise until one day he commented the a/c wasn't cooling as it should. I reluctantly removed the third seat and aux unit cover to check for the noise (its just annoying to move all of it). The noise comes from the expansion valve without a doubt and the a/c cooling air is non-existent at this point.

I checked the expansion valve with my UV light and it appears to me the expansion valve is leaking refrigerant, but I need a better set of eyes. Please see the attached photo and tell me if you believe this is die from the refrigerant.

Thanks in advance for your thoughts and guidance.

That does not look like dye leaking to me. It's usually a lot more apparent. But someone else may think otherwise...

If it's dye then it will wipe off. Clean it off, see if it comes back.

I think an A/C leak that is big enough to be heard is going to empty the system quickly. You might be hearing the flow of liquid through the expansion valve. I've read this is normal in some A/C systems.

I really don't know but I wonder if low charge in the system could lead to hearing this sound as the expansion valve attempts to compensate for the reduced available super-heat. I think the front a/c evaporator loses cooling more rapidly than the rear one when the charge is low.

If it's dye then it will wipe off. Clean it off, see if it comes back.

I think an A/C leak that is big enough to be heard is going to empty the system quickly. You might be hearing the flow of liquid through the expansion valve. I've read this is normal in some A/C systems.

I really don't know but I wonder if low charge in the system could lead to hearing this sound as the expansion valve attempts to compensate for the reduced available super-heat. I think the front a/c evaporator loses cooling more rapidly than the rear one when the charge is low.

tomj76 - good idea about cleaning it off. I will do that saturday or sunday, run the system, and see if it comes back. Regarding your thought about the low charge....interesting. Charging the system is next on the to-do list if I can figure out the rear auxiliary blower motor issue (different thread).

I have heard you can charge the a/c system with propane to check for leaks, propane is cheaper than refrigerant, but haven't tried it. Thoughts out there about doing this? Any concerns about discharging the propane?

I wouldn't use a gas like propane, at least not without some careful research. There's too much that can go wrong. The pros use Nitrogen gas because it is cheap and mostly inert in the system. The only thing better would be something like Helium or Argon, since they are totally non-reactive.

Also, a can of R-134a can be purchased for less than $10 at Walmart. Less than a can is all that is needed for leak checking.