I just tackled the warm cab air in my suncruiser this past month. It ended up being the temp switch that goes into the heater core/ac box (not sure what it's called) That prevents the box from freezing up with extended use of a/c. It was failing on mine and the power to the compressor comes directly from that switch. It is supposed to give power at +39 deg and cuts off power at 30 deg or less. Was very corroded and once it was replaced it allowed me to fill the system and works awesome now. Worth a check. Try bypassing it and see if allows the compressor to stay on to fill the system. If it does that switch may just be it. Cheap part as well.