Ambient temperature and air pressure do alter the amount of pressure for the a/c system. I looked up the system and it looks like a poorly developed system I also have - https://amzn.to/2KUFz6g - bought it on a moment of weakness (for much less $). It's not really the right thing to do the job well, and I regret getting it (for something like $10 at some car parts store). If I wanted to really do it on the cheap I'd buy one of these, at least, which can use standard R134a cans ... https://amzn.to/33VXBg9 A gizmo like this would be more appropriate... but still not ideal... Search Results For "a/c r134a manifold gauge set" (at https://www.harborfreight.com/search?q=a/c%20r134a%20manifold%20gauge%20set ) Really, if you don't have a leak, just pay a pro $100 and get it over with. If you do have a leak, pay a pro to fix it or at least get the pro to evacuate the remaining refrigerant, then find and fix the leak, then pump out the system thoroughly and refill it.