Here's one that should be simple:

When the AC is on and engaged, I will get a hissing noise for a few seconds every few minutes or so (I'm assuming each time the AC compressor engages). It seems like the likely cause of hiss is low refrigerant charge, or if I'm unlucky a faulty regulator valve. I'm hoping the dye I see on the AC lines is old and not a current leak.

Anyone have experience with the same symptom? It seems to cool reasonably well, but I've only experienced the AC in my own x308 so far, and not on a 90 degree day yet.

And if that is a likely cause, what is the usual total cost for an AC system recharge? I get the testing is simple, just checking low/high pressure, but if I take it to be tested I'd want them to recharge if it is low. The whole refrigerant system seems like one of those things that really just needs to be done properly, which requires expensive equipment and a slightly controlled environment, and so on, and is just better off being taken to a shop... Not even going to consider the refrigerant rattle cans sold at the parts stores.

 

Best to take it to a specialist, as you're already planning to he'd have more idea on the hissing.
The valve seems to be a weak point on this system, I've seen a few changed.

 

+2 on taking it to an expert.

Proper diagnosis of the system requires not only the right equipment but the right knowledge and experience. You can buy some basic equipment and learn to service A/C yourself, but you won't learn everything you need to know until you've worked on a significant number of vehicles with different types of malfunctions.

I do nearly all of my own service, including most A/C work, but when I'm not certain about the cause of poor performance, I take the vehicle to our local A/C expert, who can quickly diagnose things like a clogged expansion valve, compressor reed valve issues or compressor damage due to refrigerant overcharge (a common result of DIY service). If necessary he can evacuate the system, vacuum-test it for leaks, and recharge with the proper amount of refrigerant (by weight) and the right amount of oil to lubricate the compressor. I've rarely had to pay him more than $100 per visit and I'm sure he's saved me thousands over the years.

Cheers,

Don