Thanks for the suggestions folks. I just got the car back from the shop with new R-134, among other things. As it turned out the system was in pretty bad shape and needed a new receiver-drier, pressure switch, expansion valve and some wiring work. There was a short in the wire leading to the aux fan that took a while to find. Also ran into a problem with the compressor shutting off on decelleration--thatwas traced to a too large gap at the clutch plate. New shims and cleaning of both clutch surfaces cured that one.
All told the bill came to around $795, which I think wasn't too bad considering the time involved in diagnosis, cleaning and so forth. The car cools great. I wish I could have gotten away with $39.95, but no such luck.