I believe my system is a little low on Freon - has difficulty cooling at first on a hot day (may be 15-20 minutes before the cabin is comfortable, right center vent is cooler first. No AC codes.)

However, I’m not sure how to get the AC system set up to check the pressures. Since it’s a variable displacement compressor, it seems like you would need it wide open in order to check the high and low pressures vs. the chart - given the ambient air temp.

Am I thinking about this right? - and if so, do I just drop the controls to the lowest temp and leave the windows open so that the compressor will stay at max displacement.

 

There are so many people who think that they can solve A/C problems. You have a slow leak somewhere in the system which requires special equipment to find the location. You must seek professional help.