Your car's HVAC system needs to be set to a certain pressure range based on the ambient temperature at the time you are filling the system. Using the gauges we have ( Get yours using this Amazon Associate link- amzn.to/48wXW8N), we set the system using the following guidlines:
AMBIENT TEMP °F LOW SIDE RANGE PSI HIGH SIDE RANGE PSI
65 25-25 135-155
70 35-40 145-160
75 35-45 150-170
80 40-50 175-210
85 45-55 225-250
90 45-55 250-270
95 50-55 275-300
Don't forget to replace the caps, they act as the final seal, the valves can leak, the cap helps seal the system, just like your tire caps.