Hi, my 91 Vanagon's AC has been giving me the fits for a few years now.  Every spring I have to get it loaded with freon.  This year I had my mechanic take the evaporator down, he changed the expansion valve, put it  rebuilt compressor and fixed a very small leak in the system. The charge is again low and I don't know what to do because we have checked all joints and there is no evidence of a leak using a leak detector.  Obviously the freon is leaking somewhere.  R12 was the original fluid but since we can't get it anymore he uses something called Freeze 12 which works OK but does not cool like R12.  Any suggestions, hints, advise???
Cheers
Jaime