I know full well about the 240 blower motor debacle you reference. I still have my 1975 245 (original owner) and have replaced that motor several times. I still recall how ticked off I was after going through that torture for the first time. Being short on cash with three young kids, I figured I could lube the old Electrolux motor's exposed bearings and be done. Several days later, I learned I was mistaken! Still, at least on paper, that job seems less burdensome than the evaporator.
Did not know that the accumulator/dryer was prone to the problem you describe. Still, it's bottom is easily viewed with a UV light, and I haven't seen anything to suggest seepage. Are you suggesting that moisture from sweating may disguise this? I'm going to take a closer look.
In my case, I'm thinking evaporator because I also have a low end halogen electronic detector that came free with my vacuum pump purchase. The thing is pretty finicky, seemingly set off at times for no good reason. Still, if you look for patterns, it does seem to be a help. In my case, I pulled the water drain hose from beneath the climate control unit in the passenger compartment and turned on the blower for a few minutes to clear the housing. After turning the blower off, I put the detector's nose next to the end of the drain hose. It usually took less than a minute for the detector to start alarming, presumably due to leaking 134a from the evaporator. After putting in the leak seal, the time to start alarming took considerably longer, and sometimes it never did alarm. So there is some indication that the sealer was doing at least some good. Note that the particular sealer I used, Super Seal at about $25, claims to seal leaks originating from either rubber seals or mechanical pinholes. Some leak sealers only do seals. (Note that this product tells you to fully charge the system with 134a immediately so as not to clog the service valve.)
As to your changing out the low pressure service valve ( I assume this is the Schrader valve you're referring to), just be careful that the dye you're seeing isn't the result of charging the dye into the system. Mine was glowing green until I cleaned it up somewhat with contact cleaner. Even then I couldn't clear it all. I understand they make a special cleaner to do this, but I don't know where you get it.
One last thing on a separate issue. I just made a post in the 700 section that has to do with my 1986 740 engine noise. I know from reading many of your posts that you are very good in diagnosing such problems. I would appreciate any comments that you may have - whether good news or bad news.