Slow Leak in Auto Air Conditioner

I think we have a slow leak in the A/C system of our "Chevy". The last repair shop we took it to topped up the freon but it is toast again (just like my buns). Anyone have a recommendation for an A/C repair shop in the Chapala area or do I need to take it into Guad?

Only person around here that I know of that does A/C repair is Al Rivas in Riberas.. I state this with no recommendation, either positive or negative. chico