We had that trouble with ours last year. My husband took the valve extension off, put some wd40 on it and worked on it. Then went to test it. Held onto it with vice grips and the air shot out. I guess it was just stuck. Then he couldn't get it back on. Too tricky he said. He had the truck place where we get the oil changed do it. He said there wasn't a little black cap on it so he put one on. I hope this helps.
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