There are plenty of posts on this, suggest you try a Search. As you will see, if your evaporator is TU, you are faced with one of the most difficult and expensive jobs on the 124. So you at least need to get someone to verify the location of the leak.

And if it is the evaporator, I can't imagine spending that much effort and not going back to R12, especially in your area.

Good luck.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe