Valve stems do go bad. They should be replaced with every other tire change. Having extra cores and caps with rubber seals is a good idea.
TPMS systems have their place. My father just purchased one because last summer he had a tire go flat on the trailer while towing and since he was on a mountain road and hauling mostly empty, he had no idea one of the rear tires had gone flat and shredded. That is until a highway patrolman pulled him over because his rim was throwing off sparks.
I used to do some pretty extreme off roading when I lived in Northern Nevada and have built a couple of "rock crawling" rigs. When you are thirty miles into the back country, having a leaking valve stem is not a good thing. Especially with the dust, dirt, mud, water, etc, we would go through. To help prevent this, good metal valve stem caps with a rubber seal are a must. Also, we would coat the cores in chapstick before putting them into the valve stem (working the chapstick thoroughly throughout the core). This kept the core in a well lubricated state which helped the seal.
2015 White Hawk 25BHS Glacier Package
2004 Dodge Ram 2500, 5.7 Hemi, 5Spd Manual
2008 Arctic Cat M1000 SnoPro
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