Rick, yes, that is to be expected. When everything is working well, and the system has a full freon charge, the drop is still there, just not as obvious. Your system is short-cycling. The single-hose charging kits will only hook to the low side, that's to prevent someone from trying to fill the high side, and potentially blow up the refrigerant can. The compressor comes on, and pulls the low side down to the point the low pressure switch cuts compressor clutch power. The pressure equalizes, and the switch re-energizes the clutch, and away you go, until the pressure drops below the value set on the low pressure switch.
You probably just have a small leak somewhere. If it has been several years since the system was converted, then it could be just normal seepage. If it's only been a year, or two since the conversion, your leak may be more of a problem. As others have recommended, you may want to take the car back to the place that did the conversion (if that's possible), and have them inspect it for a leak. They can be annoying. There's not much worse than being uncomfortable in a Cadillac.