Scott is exactly right about the cheap Chinese rubber valve stems. And there is an adiditional thought to add here. Many 14" trailer tires, including Marathons, have a max inflation pressure of 50 psi, which good rubber stems can handle. But many 15" trailer tires, including Marathons, have a max inflation pressure of 65 psi. I seem to recall that rubber stems won't take this pressure, while metal stems will. WMTire would know for sure - in fact, he may be the one who clued us all in on it.
Bill
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