Since a/c problems seem to be a hot topic these days, I have a question regarding the shrader valve on the low side charging port. I was going to put on the proper R-134a fittings on the service ports (the system was retrofitted and was charged with R-134a through some homemade hoses that used the original R-12 fittings). Anyway I was attempting to add the fittings and noticed some slight hissing from the shrader valve when I remover the cap. I don't want to put the fittings on if I have to replace the shrader valve. Should it be replaced or will the cap seal the the slight leak? Does the system need to be discharged to replace the shrader valve? or can I work quickly enough to replace it and not loose the full charge of refridgerant?

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