Wish I could tell you what the pressures are for the High and Low side. But I will confirm that you need to use R-134a. I congratulate you on your recognizing that you need some thing to monitor the pressues on the high and low sides before adding anything. That being said, I believe the system holds a total of one pound. If your system is currently running, and cooling some. Dont add one pound. You will over presurize it. Bad news.