I am back in the hvac industry after a very long time away. I am working on a blast chiller that runs 404A refrig. I have just changed out the old evap. for a new one. I pressure tested the new to 200lb with nitrogen over night. Today i put a vacumm in the thing and couldn't get it to pull any lower then 20" vac. I'm using a 7cfm 2 stage pump, and pulling though a 1/2 inch to 1/4 inch set of lines on my jb gauges. ? is what am I missing, if it held pressure, it should at least pull down I would think, even if it does have a leak someplace. This unit uses a copeland compressor, both rotovalves are fully back seated. I feel like i am missing something simple, but as yet my ahaa moment isn't forthcoming. Most of my previous hvac work was with comfort cooling, so maybe i just don't know enough. Help would be appreciated.