OK - after many months of lukewarm A/C, multiple hose manifold hookups, requests for help and frustrated confusion - I have apparently solved the problem! As is the case often with me and cars the solution hit me in the face and involved stupidity - and even though the practical results are great the technical results are still confusing.
Cut to the chase - the main reason for the too-high low and too-low high pressures and the failure to accept refrigerant was - wait for it - I forgot that the charge valves at the charge ports loosen and tighten THE OPPOSITE of the manifold valves. So when I thought I was opening a valve I was actually closing it, and vice-versa! A few nights ago I watched about 8 YouTube videos on A/C problem diagnosis and one of them - only one - mentioned this little fact. I pulled out my gauge set and looked, then hit myself in the forehead a number of times.
So today I hooked everything up again, turned the valves the right way and everything was different. Low side started at 10 and high at 125 with system on. Opened the R134 can and it actually went in, can got cold and the vent temps started to drop. It took over an hour but I put in nearly 2 full cans so it must have been quite low on charge. I pulled all the hoses off and took it for a ride - WOW! On a 90 degree day with just the front unit operating I drove for about 20 minutes, starting on max AC and high blower. I quickly froze me up so I dropped down to regular AC and medium blower, still too cold so down to low blower. Even at that setting it was plenty cold, I am pretty sure it cools better now than when I first did the conversion.
So here are the confusing technical results. At a 90 degree ambient temp like I had today, the low pressure should be 45-55 psi and the high should be 250-270. The actual readings on my gauges today were 35 low and 140 high when I stopped the charge. I can't explain it and as long as the system works I don't really care. Thanks to all who commented, I really appreciate it.
