Senior Member
Join Date: Jan 2009
Location: Colorado Springs, CO
Posts: 5,974
Bikes: 2015 Charge Plug, 2007 Dahon Boardwalk, 1997 Nishiki Blazer, 1984 Nishiki International, 2006 Felt F65, 1989 Dahon Getaway V
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+1 check to see if the tube has a removable core, and if it does, is it loose?
BTW, I just now discovered that some Schrader valves also have removable cores and that removing the cores lets you inject sealant into the tube or tubeless tire without gumming up the valve.
I still run tubes on all my bikes and I have never used sealant so none of this really affects me except when I have inadvertently unscrewed and loosened a presta valve core. Accidentally loosening a Schrader valve core would be a lot less unlikely, and I have yet to do it.