Originally Posted by
BobbyG
+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.
I'm not positive, but every Schrader valve, I believe, have removable cores that can loosen up. That's why the metal Schrader valve caps have the core tightener on the top. To tighten a loose core and/or to remove and rapidly deflate.