A question for automobile air conditioning experts. I've got a 04 Toyota pick up. Last summer the air was not cooling. This summer it's slightly worse, no cool air blowing. I took it to have it serviced and the dealer couldn't find a leak. He (said) put in a bottle of dye and freon and said it should work. If and when it begins to blow warm air return and they'd check the leak (dye) with a black light. It lasted a day. I returned and they couldn't find a leak, so they recharged it and put in 2-3 bottles of dye to make sure a leak could be spotted. For the past week the a/c has been working fine. What's up with this problem? A leak, no leak? What?

Probably leaking in the dash area. Check with the black light up under the passenger side of the dash. If it isn't leaking under the hood it has to be under the dash.

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Non illegitimae carborundum

Dye is the lazy way to find a leak, especially 2, or 3 cans. I agree that the leak is most likely in the evaporator in the heater box. But it could be on the radiator side of the condensor. The bottom line is they need to spend the time to find it. There is a drain for the condensate under the hood (where the water drips when the ac is on) look for a stain there. You won't need a blacklight if you look closely, it will stain out with just a flash light.

It's almost surely leaking if they have to keep adding it..
Well, unless there is a restriction somewhere and they "think"
it's leaking out.. But it probably is leaking.. They need to use a
refrigerant leak detector. And yep, it's probably in the evaporator
coil most likely. But sometimes the compressor seals can start
leaking, so that's a possibility too. Dyes and black lights are for
pikers.. :/ Pro's use leak detectors to sniff the leak.
I don't do car work much, but I do HVAC work. I have three different
leak detectors. I still think the old GE plug in the wall model is one
of the best ever made.. I have two of those. But I also have one of
the new fangled models which runs off batteries. It's basically a piece
of junk compared to my trusty GE style units.. It works OK, but it's not
near as precise when you start narrowing down a small leak.
If they can't see any dye anywhere up front, it's probably in the evap
coil.. Which is a PIA to get to in most cases..

My wife drives a 92 Toyota with the 3.0 V6 and the AC quit blowing cool air about a season after she bought it in 03. I filled it with refrigerant and it worked great, the another season later it quit again. I couldn't find a leak anywhere and filled it again. This went on for about 4 yrs. and then it quit quiting. It has been working great for the last 3 yrs. Some of that stuff has sealer/lubricant added and I think maybe it finally stopped the leak, I hope.
Peace,
gordon

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better have that checked