The following product will let you change the valve core without you having to evacuate the system. The freon loss is bare minimum - less than that you would when removing A/C charging hoses.

Amazon.com: Mastercool (MSC81490) R-134a Valve Core Remover/Installer: Home Improvement

I used one to replace the valve cores on my '86 300E with R12 - was a 5 minute job. It's well worth the investment.

Make sure you use nylog or similar on the threads of the replacement valve core to prevent leaks.

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1986 300E (3/2002) Rear ended and totaled (10/2009)
2000 Xterra (5/2000)
1992 400E (11/2009) -Sold
1986 300E (12/2009) - Sold
2004 E500 Wagon (6/2013) - Sold