Having gone through all the BS of pulling the dash to fix the evaporator, the system still leaks refrigerant, so it appears the evaporator wasn't the source of the leak. Oh well, the evaporator was pretty clogged up with leaves and dirt so it wasn't such a waste of time.
My regular AC guy came round and we crawled over the entire system, and neither of us could find anything fluorescing under the blacklight. However, he noticed some suspicious staining on the condenser.
I borrowed his sniffer and slowly went down every single row of the condenser, and eventually found one spot where I could consistently get a reading. The sniffer is super sensitive, even moving it too fast will set it off, but it only registered the presence of refrigerant in a maybe 5mm x 5mm area of the condenser. So I feel validated that I took the time to methodically go through it row-by-row.
I've ordered a new Nissens condenser + o-rings from Skandix, which should arrive this week. My AC guy is going to come back and charge the system with nitrogen and we will let it stand for 24h to verify the fix, rather than wasting money on R134a (plus it's bad for the environment when it leaks).
