Had all three sets of O-ring on the compressor replaced and system fully charged in late March. Yesterday, the system stop working again. With black light, I found there are a trace of dye around the low pressure side connectors (black arrows on the picture). However, at the shop in teh day light, we could not see those trace marks I saw last night. And this A/C shop is not a Mercedes specialist and they have no clue what are those two tubes (marked with red arrows). Strangely, we did a large UV spot on the wall (far left red arrow).

Looks like the leak from that connector is so bad that it shoots to the wall. Is it possible? Could I just tighten those two bolts or do I need to replace the o-ring in it? Where to buy O-rings?

Looks like I will have to spend $15 to get a 609 certificate so I could buy R12 on e-Bay. I know which is the low pressure end. Do I just hook a can to that low end or do I need to jump the compressor first?

Very much appreciate your help.

Sorry, my pictures are too big to upload now. I will work them an dupload them later.