There's no problem with the vacuum or moisture in the system. He's located in Salt Lake, which is at 4226ft elevation. Air pressure and hence possible vacuum is reduced at elevation. At 4000ft air pressure is reduced to 12.2PSI from 14.7 at sea level. Maximum possible vacuum at 4000ft is 25.8 in-hg.

It might help if you post high & low side pressures at idle and 2000RPM. However, I think what you have here is a properly operating R4 a/c system. They just were not all that powerful.

Rather than refrigeration, the next place I'd look is the vacuum actuators and air doors. It's important to get 100% recirculated air in 100 degree temperatures.

Good luck.