is it true that if you put a vaccum on an a/c system that it will only hold for a while because of the design of the schrader valve? a mechanic told me he could put a vacuum on my car but he didnt have a way of putting my r12 cans in. convieniently. he said if i pulled a vaccum and didnt recharge within an hour i would lose the vaccum.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual