[Supras] Air conditioner problem

[JAMES]

  For the last two summers I have had my 89T charged with FREEZE 12. I drive it daily but if there was a cool spell
 like in the fall and I went a couple of days with out using it, It would quit blowing cold.
  So last week I took it back to my toyota dealer and they said that this time they would put in a dye and 
 charge it back up and when it leaked out this time, they would find it. 
 They called me at lunch time and said that they found a leak as soon as they started charging it up.
 They replaced an o-ring and said that if "I am lucky, maybe that's the only leak that it has".
 I picked up the car at 5:30p.m. after paying $152.00. As soon as I left the parking lot, I noticed that it was blowing
 hot air. They were closing but I called the service dept and told them.They said that it was blowing cold when they
 finished it around 1:00p.m This was on friday afternoon. They told me to bring it back monday morning.
 I took it back monday morning and they have had it for a week. They said that they can not find any leak in the
 front of the car. They said that it must be leaking under the dash and if they remove the dash to try to find it,it could
 cost up to & $600.00 and they would not guarantee it. They said that if I wanted them to do this, they would apply the
 $152.00 toward the cost of doing this and if I don't have it done I would lose my $152.00.
   To me I think  I am getting @#*!ed. They have always done my service work and I have always been pleased
  with their work. I know air conditioner work can be a pain.   
    What I wanting to know from you guys is: Are they a certain thing that is common to go bad? A certain part that may not
  require removing the dash. Or maybe access it by removing the glovebox? I read on here a few weeks ago, 
  to add oil when you charge it up, could this help? Could I maybe convert it over to R134A and add the kind that has stop
 leak in it? 
   The dealer said that they did not have a service manual on this old of a car.    
  I love my Supra, but I need cold air.  Any ideas would be appreciated.
                  Thanks,
                              James