[QUOTE=cy;13238761]just changed out a hi/lo pressure switch on a semi-hermetic compressor using this tactic. get everything ready for the switch, then crack system open. plug release port with finger, then quickly switch out fittings.

very little refrigerant lost with zero contamination getting into system because positive pressure was never lost. much less refrigerant lost vs pumping down system.
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CY - Why would you pump down the system? Why not just isolate the compressor instead of hoping you can change the control out fast enough. If it's a semi-hermetic it has isolation valves....just wondering.