Are your hoses fittings R134 or R12 style? If they are the R-12 style, or just plain old, you should replace the barrier hi/lo hoses with new and the drier as well. Was not mentioned if you had done that. Given your scope of work and investment thus far, would be worth doing it so it will last another decade w/o issue.

When you draw it into vacuum and fill, plus the can pressure, you should be able to get to 20-25psi without the compressor. I believe the pressure switch on the compressor will not engage until near 20psi is detected. From there the comp will aid in filling to spec depending on the gas you are using.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)