I don't know what a "dry nitrogen test" is? Do you mean you are pressurizing the system?
I have all sorts of equipment for detecting AC leaks and sometimes none of them work.
I often find the one thing that often finds the problem when other more common tests do not? Hook an air-chuck to a charge-port. Pressurize to 150 lbs. and then just go over everything with a spray bottom of soap-water and look for bubbles.
What happens when you put a vacuum on it? Does it hold full vacuum for day with no loss?
I find that with some problem AC systems, no vacuum leak shows and not pressure leak shows when the system is working. But once you turn off the engine and let the low-side pressurize - sometimes that leak will show with dye but that is only at around 60 PSI. Thus why my test with 150 PSI from my air-compressor often finds the problem when other tests do not.
One other comment. When 134A first came into use, it was believed that the rubber AC hoses made for R12 would leak with the smaller molecules in the 134A. So since then, all new rubber hoses are "barrier" hoses with a special lining inside. Not sure if you have new hoses made for 134A or not. Now adays all new AC hose is "barrier" hose so the descriptive word is often not even used anymore.