AC Problems

I am trying to repair the AC on my 2001 Ford Expedition, Eddie Bauer Edition w/5.4L. System has not worked for the past 3-4 years. With my neighbor's help and using pressurized CO2, a leak was detected through a valve (pop off valve?) Located on the hose block fitting at the compressor. The valve was replaced with a new unit and the AC was vacuumed and filled with new R134a refrigerant. System started to cool, but AC compressor caused engine to rev erratically. Refrigerant was left in the system for a week before being evacuated for reuse. In other words, the system held the charge even though the compressor needed to be replaced. Next, the system was evacuated and the AC compressor was replaced with a new Ford factory unit, the system was vacuumed, 20 oz of PAG oil was added and then the system was recharged using the gas collected earlier. Amount of refrigerant was not accurately added to the system only estimated using a gage set. System did produce cold air but after 4 days changed to warm air. System has had dye added several years ago, so used black light to look for a leak at the hose block fitting at the compressor - noticed a leak at the valve in the hose block that was replaced new earlier - dye showed up on the end of the valve indicating to me that the valve had opened and let refrigerant escape from the system. I suspect that the charge has leaked through this valve. No other components in the system were replaced. My neighbor is not able to assist me, but I have a gage set that I can connect to the vehicle. What caused the valve on the hose block to discharge and what should be done next?