Expansion valve failure?

The system needs to be completely evacuated under vacuum before any refrigerant is added. I always assemble the system, apply vacuum. and leave it overnight to check for leaks. The system needs to be completely evacuated of non condensable materials. A new filter/dryer is a must. While the system is under vacuum, be sure to rotate the compressor by hand to ensure all passages have been exposed to vacuum to ensure all the moisture is removed. It's not as hard as it sounds, and many auto parts stores, like Auto Zone, will loan you a vacuum pump. The moisture must be removed from the system, as 67Dart274 recommends, is correct for maximum efficiency. If you have problems, post again. There are many experts on this site who'll help you.