They won't leak when they are new. But over time, that nylon seal will only need to get a "groove" from a foreign particle, such as a small shaving of metal etc, and it will. In my previous line of work ( I am now retired) I dealt with this type of valve on a daily basis and eventually they all leaked.
But that could be after 5 or 6 years, or even 8 -10, but also conversely after 6-12 months. In 98% of cases, it was damage from foreign material that caused the leak.
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