There are no "normal" pressures, especially in a vehicle. They are
determined by outside air temp and engine RPM, to name a couple. The system
should be checked with the RPM's at about 2500- that's considered average
cruising RPM. If you can get 50 deg. air coming out of the dash vents at
that RPM, the problem is not in the charge, but more likely an air flow
issue. I used to check the cars I worked on with the fan on medium and the
fresh air vent open. At those settings and at 2500 RPM, the frost line would
get to about 18-24 inches from the compressor (depending on the car).
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