aztec airbrush

I think i missexplained my problem that i have with my airbrush,so i will try to put the right words on the right things.(that what happen when english is not your first language)When i use the gravity cup feeder in single action mode the mode flow out the cup without i press the trigger the paint flow out in big drops it is like they are i leak in the nozzle or the airbrush. I try new nozzles but the problem appear again.This problem doesnt appear when i use the double-action mode. I follow the instruction very carefully , i thin my paint correctly and i clean my airbrush very carefully afer each use and i dont diasassemble the paint nozzle.