I have a 1991, all original, with R12. My Ac dryer was leaking from the viewport seal, bubbling slowly. I got a new dryer. I evacuated the system, and left the gauges on over the weekend and the pressure kept at -30inHG. Now a yearish later, the a/c has lost pressure again. It still had some pressure in it but not enough to turn on the compressor. I evacuated again, on Monday, now on Thursday it is still holding again at -30inGH. Should I leave it on for a few more days? I guess my question is, Can an O ring hold pressure at negative and somehow lose pressure at positive when the system is pressurized?