The A/C system must be evacuated before you replace the refridgerant. Replace your parts. Once the system is buttoned down, have it evacuated with a professional . You may want to have it filled with refridgerant at the same time. It will proably cost you the same or less than doing it yourself. They also have professional A/C gauges to diagnose a problem when the system is brought back to life, especially when things don't go right.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T