for past 3 years have had to replace freon. First effort to use dye to find leak failed. Now mech says it's a compressor leak. How does he know that. Wouldn't the dye have shown the leak?

Speaking of dye - if there is a very slow leak, wouldn't it make sense to charge the system, including the dye, and then come back after a few weeks to look for the dye trace, giving it time to build up? or is the dye only visible when damp.

car has less than 30,000 miles.

Thanks for your help.

pete