You are so right Matt. If one were to put a gauge on a 30lb cylinder of refrigerant the pressure in it would not matter whether it had 20lbs of refrigerant left or five pounds. As long as there is liquid the presure is governed by the relationship PV=nRT. Basically this states that at a given volume and temp the equilibrium will set the pressure the same always. Thus the pressure on a static system is the same no matter how much refrigerant is in it as long as there is liquid present.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician