Tomorrow I'll run the car up on ramps and crawl around underneath to see if I can find the hi side fitting. Of course once I identify it it'll be obvious.

The system wasn't evacuated. It was reading 12 gpsi when warmed up with the ac running 10-15 minutes. From the MB 123 cd info, which gives a low side pressure/temperature graph for normal operation, I was looking to get to a pressure of something over 2 bar, about 30 psi, when I hooked the charging cannister up; I saw an immediate rise to 50-60 gpsi, which might mean I have overcharged it? There was little flow from the cannister to the system.

Obviously I was looking for a quick and easy fix, without fully understanding the system. Previous service records indicated an annual or semiannual need for additional refrigerant, and I hoped this would quickly put me back in business, while I was learning more details of ac operation. I'm not sure where I stand, now.