schrader valve failure

I installedAC in my '96 Ranger 3 years ago - and it worked good for 3 years. It didn't work this spring so I topped it up with 134A and die, and it worked for about 2 weeks. I went at it with the UV light and found a big stain at the low pressure service valve. Had it into the shop today and replaced the shrader valve - no other leaks. How common is it for a shrader valve to fail "in service" (not while being charged or drawn down)?
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On 7/14/2016 11:37 PM, snipped-for-privacy@snyder.on.ca wrote:

With central AC systems, the parts houses sell replacement cores. So, there must be some demand for them. How common? I'm not sure. Wish I could be more help.
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On Fri, 15 Jul 2016 09:25:32 -0400, Stormin Mormon

I think for the ninety six cents they cost it would be a good idea to replace them every time a system is "opened" (refrigerant removed and system drawn down or pressure tested) . There ARE tools made to replace the core without emptying the system -so I guess it's not an uncommon problem.
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