As the previous replies indicate, the first thing you need to know is why the AC doesn't work. Is the compressor turning? Does the system have pressure?

The fact that you had it recharged would tell me that it was low, but not necessarily. Many techs just pull out the contents and recharged by weight. They may not know how much was in there to start with. Did they tell you that the system was so low it wasn't working? And did the system work after they recharged it? YES! one should definitely run the system with the dye in it. The dye is in the oil and if the system just sits the oil will only touch the bottom of each container (evap, cond, receiver, compressor, and hoses. If the leak is at the top the dye would not show anything.

During normal activity the oil and refrigerant blend and flow as one through the system, coating all surfaces and allowing a good indication. one should run it till the performance is reduced and then check with a black light for leaks. WE normally see evaporator leaks as stains that run over the sides of the trans from the drains.

Obviously, if the evap is the problem full AC will be restored with a recharge. If it doesn't that isn't the problem.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician