Dye usually works, but not always. When dye fails, it is best to put in 2 oz. of R22 and pressurize with nitrogen then use an electronic leak detector. An electronic leak detector is very sensitive R22, that's why it works so well.

Don't even think about replacing evaporator unless YOU ARE CERTAIN that it is the leak source. They are not destined to fail as some people say. Mine has the original evaporator and is blowing 38 degree air.

The expansion valve is a somewhat common leak source. They will leak at the top where the diaphragm meets the block. It is hard to see the dye. I could barely see dye on mine, but there was an oily film around it which is another clue of a leak. Once I took it out and shined the black light on it, it was very obviously a considerable leak source.

Good luck,