I have a very small leak in my compressor. I also have a set of HF gauges. First thing, when I add freon, I look at an ambient temperature chart to see where the pressures need to be. Also I find that with 134A, it is much more accurate to read the HP side. Also, the pressure will come up as the RPMs come up. So, if at 90° F the chart says 240 to 275 for example, then at idle its at 240, and at say 2000 RPMs its at 275, your golden. Those numbers are just an example, but you get the picture.