Over past 20 years. You note you released some with a 2 psi reading so I assume at that time most gone. Did you release any other over the past 20 years. I have owned a 92 NA since 92 with auto climate control with no issues or R12 added since new. So if system sealed, compressor sealed tight and all fittings and hoses in great shape R12 should leave as evidenced by my AC that blows as just as cold as new.