Toyota Repair: air conditioning, freon, time air

Question
1999 Camry.  We have charged the A/c system 4 times.  Each time, air is cool for 2-3 months.  They have never been able to diagnose a leak.  Then the a/c light on the switch starts to blink and the air ceases to be cool.  What can be wrong?

Answer
Just because they can't find a leak doesn't mean there isn't one, what I would suggest is evacuating the system again then add a flourescent dye into the system and then recharging with the correct amount of freon, once the freon has leaked out again the leak can be found with a UV light, you'll have to find a shop that knows what I'm talking about, the leak may not be external and it could be inside at the evaporator or the expansion valve, one other thing, when they remove the freon the machine will give a reading how much freon is recovered, the system holds about 1.4lbs of freon, if all of it is recovered there is no leak, if less is recovered then it's obvious that the remainder had to go somewhere, which means there is a leak, I don't know who you're dealing with now but make sure that they know what they are doing and are not going to just refill it again and you still have the same problem, let me know how this goes.