At one time Freon was cheap and a 2oz. per year leak was acceptable in a 4-5 lb. System. Today systems are smaller and 1/2oz makes a big difference in performance. Good halogen leak detectors will show most system leaks, under static conditions, however some will leak only when the system is in operation. This is where the insertion of a florescent dye can help find the leak. Note, to see the dye stain you need a UV lamp and the leak has to be where the light can reach it. Most car manufactures approve the use of trace dyes and some OEM's have been inserting trace dyes on the assembly line. Dye can be inserted either when charging or when the system is charged. The amount of dye used is less than 1/3 oz and remains in the system even if the total charge is lost. The dye stains the oil not the Freon and only flushing will remove it.