Glycol chiller expansion tank

Can someone give me an answer to two of my questions?

I operate a single stage ammonia system and we use a glycol chiller to maintain temperatures for a banana room ripening operation. My suction pressure runs at 36 pounds and the chiller has a johnson controls modulating suction valve operated off of a bulb t-stat on the supply glycol line from the chiller. The chiller is also fed with high pressure liquid. That sums it up.

I have a expansion tank below the chiller (red in the picture) which is pipped to the air seperator on the return glycol line. My site glass on the expansion tank is nearly full of glycol and the pressure gauge reads zero. What should I be seeing here and is this operating properly. Mechanically speaking how does my expansion tank function in this setup, is it similar to a hydronic loop for heating? Oh refrigeration gods please tell me what you think. Thanks!