OK, just talked to Lori. She had this to say. "Any time the air conditioning system is opened (exposed to the atmosphere), the dryer must be replaced, especially in Florida! The problem isn't leftover R12 but oil and moisture contamination. Also, the dryer should be the very last item to be replaced as it is shipped sealed." She also agrees with Chuck, that the expansion valve can be compounding the problem and should also be replaced, since the system has to be evacuated again anyway.
So, replace your expansion valve, then your dryer keeping in mind to lube all of the o-rings. Evacuate for an hour, according to her, due to the moist Florida enviorment, then see if it will hold vacuum for another half hour. Now, charge with 134a.
She is an acclomplished tecnician who, back in the day even had a hand in installing a hemi in a '87 7-series BMW. I know this sounds like bullshit, but I just got back from Witchita (she had her 25th high school reunion) and visited the shop that this was done in. It's the "European Car Company" and I saw some stuff that blew my mind. Full cage BMW 2002's set up for SCCA racing, a Bentley under restoration, a Porche with a full cage, aftermarket fuel injection and a turbo as big as my head (they had to mount it where the passenger's feet would be so as not to interfere with the front suspension) and a sweet Alfa-Romeo (red, of course) with giant side draft Webers that almost made me cry when they fired it up for us.
Her dad has pictures somwhere of the 7-series with the hemi. When I get 'em, I'll post 'em.